Documentation: dt: binding: atmel-sama5d4-wdt: for SAMA5D4 watchdog driver

The compatible "atmel,sama5d4-wdt" supports the SAMA5D4 watchdog driver
and the watchdog's WDT_MR register can be written more than once.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>