beaglebone blue pwm outputs

used by motor outputs:
ehrpwm1a: M1
ehrpwm1b: M2
ehrpwm2a: M3
ehrpwm2b: M4


connector   pin         pwm output  config-pin command
---------   --------    ----------  --------------------
GPS         3 (rx)      ehrpwm0a    config-pin P9_22 pwm
GPS         4 (tx)      ehrpwm0b    config-pin P9_21 pwm
SPI1.1+2    4 (miso)    ehrpwm0b    config-pin P9_29 pwm
SPI1.1+2    5 (sck)     ehrpwm0a    config-pin P9_31 pwm
GP0         6           ecap2       config-pin P9_28 pwm2

pwm mode available in theory but not currently supported:
SPI1.2      6 (ss2)     ecap0       ### pwm mode missing in dt
UT0         3 (rx)      ecap2       ### not configurable (debug console)
UT0         4 (tx)      ecap1       ### not configurable (debug console)