Add REG_ID_OFF to power off picocalc

add button callback when short press Power key of picocalc

Code/picocalc_keyboard/reg.h

@@ -19,6 +19,7 @@ enum reg_id
   REG_ID_BAT = 0x0b,// battery
   REG_ID_C64_MTX = 0x0c,// read c64 matrix
   REG_ID_C64_JS = 0x0d, // joystick io bits
+  REG_ID_OFF = 0x0e, // POWER OFF
   REG_ID_LAST,
 };