watchdog: coh901327: convert to use watchdog core

This converts the COH901327 watchdog to use the watchdog core.
I followed Wolframs document, looked at some other drivers and
tested it on the U300.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>