watchdog: at91sam9: use endian agnostic IO

Use endian agnostic IO functions for the watchdog driver for when it
is enabled on ATSAMA5D36 devices running in big endian.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>