modules: hal_nordic: nrfx: nrfx_glue: Improve GPIOTE_CHANNELS_USED

Channels owned by a child core shall also be included in the mask of
used channels (channels that cannot be allocated by the GPIOTE channel
allocator).

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