boards: nordic: Add to GRTC missing child-owned-channels allocation

The child-owned-channels property of GRTC is used by nrfx_grtc
driver to exclude channels for common pool of channels allowed
for dynamic allocation. That is sort-of workaround for missing
property that allowes to remove some channels from the pool.

There are also not aligned GRTC IRQs for nRF54H20 and nRF54L15.
Only one of avaialbe IRQs was added to GRTC in DTS whereas
there should be two. That allows to find second IRQ by other
drivers that use GRTC peripehral.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>