Bluetooth: controller: LLL: DF add initialization of ant switching GPIOs

Add initialization of GPIOs that are used for antenna switching.
GPIOs provided to DFE extension in Radio peripheral, to drive
antenna switching, should be initialized in GPIO peripheral beforehand.
The initialization is optional and may be disabled by Kconfig.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>