gpio: Add MT7621 GPIO support

This patch adds GPIO support for the Mediatek MT7621 SoC, tested on
MT7688 (Gardena smart-gateway). The driver is loosly based on the
Linux kernel version.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
[fixed checkpatch.pl warnings: Prefer 'unsigned int' to bare use of 'unsigned']
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>