Bluetooth: controller: add FEM offset indirection

Hide the CONFIG_BT_CTLR_GPIO_..._OFFSET Kconfig macros behind
equivalent ones in the HAL_RADIO_ namespace. This removes any direct
usage of the Kconfig macros from the controller HALs.

That will be convenient when we are getting these values from
devicetree properties instead.

Notice that the "PDN/CSN offset" is dropping the "CSN" portion of the
name: CONFIG_BT_CTLR_GPIO_PDN_CSN_OFFSET is now
HAL_RADIO_GPIO_PDN_OFFSET. This is because the DT binding for nRF21540
has a pre-existing 'pdn-settle-time-us' property we're going to use to
replace the Kconfig option.

Other than that, the name changes are self-explanatory.

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