modules: hal_nordic: Use common nrfx_config section for GRTC

Move code that prepares `NRFX_CONFIG_GRTC_*` definitions based on
information from devicetree from the nRF54L15 nrfx_config header
to the global one, so that the code can be used by nRF54H20, too.

The checks that validate owned-channels and child-owned-channels
DT properties are moved to the nrf_grtc_timer driver so that
the global nrfx_config is not polluted unnecessarily.

The default values in nrfx_config_nrf54l15_enga_application.h
are restored to those from the corresponding template file.

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