firmware: xilinx: Use of_genpd_sync_state()

To make sure genpd tries to power off unused PM domains, let's call
of_genpd_sync_state() from our own ->sync_state() callback.

Cc: Michal Simek <michal.simek@amd.com>
Tested-by: Hiago De Franco <hiago.franco@toradex.com> # Colibri iMX8X
Tested-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com> # TI AM62A,Xilinx ZynqMP ZCU106
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20250701114733.636510-17-ulf.hansson@linaro.org