gpio: modepin: Add driver support for modepin GPIO controller

This patch adds driver support for the zynqmp modepin GPIO controller.
GPIO modepin driver set and get the value and status of the PS_MODE pin,
based on device-tree pin configuration. These four mode pins are
configurable as input/output. The mode pin has a control register, which
have lower four-bits [0:3] are configurable as input/output, next four-bits
can be used for reading the data as input[4:7], and next setting the
output pin state output[8:11]. By default value of mode pin register is 0.

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