watchdog: meson-wdt: add support for the watchdog on Meson8 and Meson8m2

The watchdog IP block on Meson8 and Meson8m2 is already supported by the
existing meson-wdt driver. Meson8 uses the same register bits as Meson6,
while the newer Meson8m2 SoC uses the same register bits as Meson8b.

Currently watchdog support on Meson8 SoC already works because
meson8.dtsi simply uses the "amlogic,meson6-wdt" compatible. Adding a
separate compatible for Meson8 makes this more explicit though.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>