pac55xx: gpio: Initial Implementation of PAC55xx GPIO Driver

* Conforms mostly to the STM32 GPIO API where possible.
* Supports pin configuration (direction, pull-up/down, etc.) as well
  as pinmux configuration.
* Supports set/clear/get operations to the GPIO port/pins.
* Created base doxy header and groups to align with existing formatting.

lib/pac55xx/Makefile

@@ -35,6 +35,8 @@ TGT_CFLAGS  += $(DEBUG_FLAGS)
 TGT_CFLAGS  += $(STANDARD_FLAGS)
 ARFLAGS		= rcs
 
+OBJS += gpio.o
+
 VPATH += ../cm3