firmware: xilinx: Don't share zynqmp_pm_init_finalize()

As there no longer any users outside the zynqmp firmware driver of
zynqmp_pm_init_finalize(), let's turn into a local static function.

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/20250701114733.636510-16-ulf.hansson@linaro.org