watchdog: ep93xx: Convert the watchdog driver into a platform device.

Convert the ep93xx watchdog driver into a platform device and
remove it's dependency on <mach/hardware.h>.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Ryan Mallon <rmallon@gmail.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Reviewed-by: Mika Westerberg <mika.westerberg@iki.fi>
Acked-by: Arnd Bergmann <arnd@arndb.de>