watchdog: meson: Enable meson SoC specific data

With this patch we refactor the driver code to enable watchdog support
for all platforms based on Amlogic meson SoCs.
The new default timeout is also now chosen considering the maximum
timeout allowed by the SoC.

Signed-off-by: Carlo Caione <carlo@endlessm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>