gpio: add driver for Mellanox BlueField GPIO controller

This patch adds support for the GPIO controller used by Mellanox
BlueField SOCs.

Reviewed-by: David Woods <dwoods@mellanox.com>
Signed-off-by: Shravan Kumar Ramani <sramani@mellanox.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>