soc: nordic: nrf54h: Disable GPD for MCUBoot

Disable GPD for MCUBoot build, as it cannot be
reinitialized later in application (SDFW does not
support reinitialization).

Also, remove the GPD disabling from the mcumgr sample
for nRF54H20 iron board app - it was the reinitialization
that caused problems.

Signed-off-by: Michal Kozikowski <michal.kozikowski@nordicsemi.no>