power_domain: nrfs_gdpwr: patch init sync

The nrfs_gdpwr driver has a two stage init, which requires tracking
an "expected" state of the power domain, to be applied once the
ipc service needed to control the power domain is ready. This was
tracked with the "off" member of struct domain_data.

So, "off" was initialized to false, but, should actually be
initialized to true, since by default, if nothing needs the power
domain, it should be turned off when ipc service is ready.

To fix this, rename "off" member to "on" and adjust logic.

Additionally, define the correct struct domain_data in the
DOMAIN_DEFINE() macro. Can save 2 bytes. This did not cause any
issue since we typecast it anyway, and its size is large enough.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>