watchdog: armada_37xx: Convert to official DT bindings

Official DT bindings have only one reg property: watchdog address space.
Convert armada-37xx-wdt.c driver to offical DT bindings and access sel_reg
register via MVEBU_REGISTER() macro, as its value (required by U-Boot
driver) is not in DT yet. In later stage can be driver cleaned to not use
it.

This change would allow U-Boot to use A3720 watchdog DTS structure from
Linux kernel.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>