watchdog: Convert jz4740_wdt driver to watchdog core

This patch converts jz4740_wdt driver to use watchdog core APIs.
Also use devm_* APIs to save a few error handling code.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>