Bluetooth: controller: add PA/LNA existence indirection

Add indirection macros for the ifdefs that test for the existence of
PA and LNA pins.

This will be convenient when the pins are configured in devicetree,
and the Kconfig symbols will no longer be around.

No functional changes expected.

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