wdt: Add MT7621 watchdog driver

This patch adds watchdog support for the Mediatek MT7621 SoC. 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 merge conflict in drivers/watchdog/Kconfig]
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>