documentclass{memoir}
usepackage{enumitem}
usepackage{longtable}
usepackage{booktabs}

newcommand{fuda}[2]{%
Functions:%
begin{itemize}[nosep]
#1%
end{itemize}
Data:%
begin{itemize}[nosep]
#2%
end{itemize}
}
begin{document}
begin{longtable}{p{0.3textwidth}|p{0.7textwidth}}
caption[List of components]{List of functions and data allocated to each component} label{d_plan} \
toprule
textbf{Components} & textbf{Functions and Data} \midrule
textbf{Registration} & fuda{
item Register capabilities of the device
item Unregister capabilities of the device
item Register context representation
}{
item  Interface List
item Capability List
item  Protocol List
item Representation List}
\bottomrule
end{longtable}
end{document}

unskip strut par egroup hfil

documentclass{article}
usepackage{enumitem}
usepackage{longtable}
usepackage{booktabs}

newcommand{fuda}[2]{%
Functions:%
begin{itemize}[nosep]
#1%
end{itemize}
Data:%
begin{itemize}[nosep]
#2%
end{itemize}}

begin{document}

begin{longtable}{p{0.25textwidth}|p{0.7textwidth}}
caption[List of components]{List of functions and data allocated to each component} label{d_plan} \
toprule
textbf{Components} & textbf{Functions and Data} \midrule
textbf{Registration} &fuda{item Register capabilities of the device
item Unregister capabilities of the device
item Register context representation
}{item  Interface List
item Capability List
item  Protocol List
item Representation List}vspace*{-baselineskip}
\bottomrule
end{longtable}

end{document}

vspace*{-baselineskip}leavevmode

documentclass{memoir}
usepackage{enumitem}
usepackage{longtable}
usepackage{booktabs}

newcommand{fuda}[2]{%
Functions:%
begin{itemize}[nosep]
#1%
end{itemize}
Data:%
begin{itemize}[nosep]
#2%
end{itemize}}

begin{document}

begin{longtable}{p{0.25textwidth}|p{0.7textwidth}}
caption[List of components]{List of functions and data allocated to each component} label{d_plan} \
toprule
textbf{Components} & textbf{Functions and Data} \midrule
textbf{Registration} &fuda{item Register capabilities of the device
item Unregister capabilities of the device
item Register context representation
}{item  Interface List
item Capability List
item  Protocol List
item Representation List}
vspace{-baselineskip}mbox{}
\bottomrule
end{longtable}

end{document}

newcommand{fuda}[2]{%
begin{minipage}[t]{linewidth}
Functions:%
begin{itemize}[nosep]
#1%
end{itemize}
Data:%
begin{itemize}[nosep]
#2%
end{itemize}
end{minipage}}

documentclass{memoir}
usepackage{enumitem}
usepackage{longtable}
usepackage{booktabs}

newcommand{fuda}[2]{%
begin{minipage}[t]{linewidth}
Functions:%
begin{itemize}[nosep]
#1%
end{itemize}
Data:%
begin{itemize}[nosep]
#2%
end{itemize}
end{minipage}}

begin{document}

begin{longtable}{p{0.25textwidth}|p{0.7textwidth}}
caption[List of components]{List of functions and data allocated to each component} label{d_plan} \
toprule
textbf{Components} & textbf{Functions and Data} \midrule
textbf{Registration} &fuda{item Register capabilities of the device
item Unregister capabilities of the device
item Register context representation
}{item  Interface List
item Capability List
item  Protocol List
item Representation List}
\bottomrule
end{longtable}

end{document}