mips: mt76xx: Enable watchdog support

This patch enables and starts the watchdog on the MT7620 platform.
Currently the WD timeout is configured to 60 seconds.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
[fixed build error due to missing function prototype arch_misc_init]
Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>