imx: wdog: correct wcr register settings

We should not simple use "writew(WCR_WDE, &wdog->wcr)" to set
wcr, since this will override bits set before reset_cpu.

Use clrsetbits_le16 instead of writew to fix this issue.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>