dts: nrf54h20: add grtc channel 15 to the pool.

Adds channel 15 to the pool of grtc channels available
for allocation (i.e. with 'z_nrf_grtc_timer_chan_alloc')
on nRF54H20.

The change is motivated by lack of available channels
for the nrf_802154_timestamper when building for nRF54H20.

Signed-off-by: Piotr Koziar <piotr.koziar@nordicsemi.no>