firmware: xilinx: Add system shutdown API interface

Add system shutdown API interface which asks firmware to
perform system shutdown/restart.

Signed-off-by: Rajan Vaja <rajan.vaja@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Jolly Shah <jolly.shah@xilinx.com>
Link: https://lore.kernel.org/r/1587761887-4279-24-git-send-email-jolly.shah@xilinx.com


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