dts: nordic: update UICR definition on nrf54h20

With IronSide SE there is only one defined UICR which is at
the location of the APPLICATION UICR. Update the devicetree
definition accordingly, and use the "nordic,nrf-uicr" compatible
on the node since the domain distinction added by the v2 compatible
is no longer relevant.

Signed-off-by: Jonathan Nilsen <jonathan.nilsen@nordicsemi.no>