watchdog: add amlogic watchdog support

Add support for hardware watchdog timer for Amlogic SoCs.
This driver has been heavily inspired by his Linux equivalent
(meson_gxbb_wdt.c).

Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>

Signed-off-by: Philippe Boos <pboos@baylibre.com>
Reviewed-by: Stefan Roese <sr@denx.de>