zynqmp: gpio: Add support for zynqmp gpio modepin driver

ZynqMP modepin driver has capability to get/set/check status of modepin
gpios. These modepins are accessed using xilinx firmware. In modepin
register, [3:0] bits set direction, [7:4] bits read IO, [11:8] bits
set/clear IO.

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/2d802d98fd56d95d764532a33e844d935e0cebb3.1635505900.git.michal.simek@xilinx.com