drivers: gpio: nrf: Add mask to track GPIOTE usage

So far, register state was used to determine if GPIOTE channel is busy.
This leads to issues if channel is used in more customized way after
allocation. In particular, if it temporarly disabled since disabled
channel is treated as available and can be allocated to another user.

Added additional mask which tracks allocated channels. After allocation
user can reconfigure the channel.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>