Bluetooth: Controller: nrf5: Allocate GPIOTE channels

Allocate the GPIOTE channels that the Bluetooth Controller needs to use
with the dedicated function provided by the nrfx_gpiote driver instead
of using hard-coded indexes of these channels, as now, when the driver
supports multiple GPIOTE instances, it would be much more difficult to
properly inform the driver which channels should be reserved.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>