Bluetooth: controller: add GPIO polarity indirection

Hide the CONFIG_BT_CTLR_GPIO_..._POL_INV Kconfig macros behind
equivalent ones in the HAL_RADIO_ namespace. This removes any direct
usage of the Kconfig macros from the controller HAL ifdeffery, except
for the usage in radio.c where the actual work is done.

Add "NRF21540" in the macros that are specific to that FEM.

That will be convenient when we are getting the GPIO polarity from
devicetree GPIO flags instead.

No functional changes expected.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>