rt2800: initial watchdog implementation

Add watchdog for rt2800 devices. For now it only detect hung
and print error.

[Note: I verified that printing messages from process context is
fine on MT7620 (WT3020) platform that have problem when printk
is called from interrupt context].

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>