ARM: dts: i.MX27: Using "wdog_ipg_gate" clock source for watchdog

Patch replaces "dummy" clock source for watchdog with "wdog_ipg_gate".

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>