ARM: mvebu: Add A375/A380 watchdog binding documentation

This commit documents the new support for "marvell,armada-{375,380}-wdt"
compatible strings and the extra 'reg' entry requirement.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>