firmware: xilinx: Add APIs to read/write GGS/PGGS registers

Add APIs to read/write PGGS and GGS registers.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Signed-off-by: Jolly Shah <jolly.shah@xilinx.com>
Link: https://lore.kernel.org/r/1587761887-4279-22-git-send-email-jolly.shah@xilinx.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>