firmware: firmware-zynqmp: Add zynqmp_pm_set_gem_config api

Add zynqmp_pm_set_gem_config() api to configure GEM secure registers.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/f69e32355c6a6be7d2780663353c52757530207d.1648631275.git.michal.simek@xilinx.com