firmware: zynqmp: Add MMIO read and write support for PS_MODE pin

firmware: zynqmp: Add MMIO read and write support for PS_MODE pin

Add Xilinx ZynqMP firmware MMIO APIs support to set and get PS_MODE
pins value and status. These APIs create an interface path between
mode pin controller driver and low-level API to access GPIO pins.

Signed-off-by: Piyush Mehta <piyush.mehta@xilinx.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>