gpio-matrix.dts

 /dts-v1/;
    /plugin/;
    / {
           compatible = "brcm,bcm2835", "brcm,bcm2708", "brcm,bcm2709";

           fragment@0 {
              target-path = "/";
              __overlay__ {
                 keypad: gpio-matrix {
                    compatible = "gpio-matrix-keypad";
                    debounce-delay-ms = <10>;
                    col-scan-delay-us = <10>;
                    /*
                       try to use GPIO only lines
                       to keep SPI and I2C usable
                    */
                    row-gpios = <&gpio 21 0    // 0 &gpio 27 0
                                 &gpio 20 0    // 1 &gpio 22 0
                                 &gpio 26 0    // 2 &gpio 23 0
                                 &gpio 25 0>;  // 3 &gpio 24 0

                    col-gpios = <&gpio 27 0    // 0 &gpio 21 0
                                 &gpio 22 0    // 1 &gpio 20 0
                                 &gpio 23 0    // 2 &gpio 26 0
                                 &gpio 24 0>;  // 3 &gpio 25 0
                    /*
                      Keycodes from /usr/include/linux/input-event-codes.h
                      converted to hex using printf '%02x\n'
                    */

                    linux,keymap = <
                                 // Keymap Entry     col  row  input  code  hex
                                    0x00000069    // col0 row0 KEY_A  30    69
                                    0x10000070    // col0 row1 KEY_B  48    70
                                    0x20000071    // col0 row2 KEY_C  46    71
                                    0x30000072    // col0 row3 KEY_D  32    72

                                    0x01000074    // col1 row0 KEY_E  18    74
                                    0x11000075    // col1 row1 KEY_F  33    75
                                    0x21000076    // col1 row2 KEY_G  34    76
                                    0x31000077    // col1 row3 KEY_H  35    77

                                    0x02000079    // col2 row0 KEY_I  23    79
                                    0x12000080    // col2 row1 KEY_J  36    80
                                    0x22000081    // col2 row2 KEY_K  37    81
                                    0x32000082    // col2 row3 KEY_L  38    82

                                    0x03000084    // col3 row0 KEY_LEFTSHIFT  42 84
                                    0x13000085    // col3 row1 KEY_N          49 85
                                    0x23000086    // col3 row2 KEY_O          24 86
                                    0x33000087>;  // col3 row3 KEY_ENTER      28 87
                 };
              };
           };
      };