ARM: EXYNOS: Clear SYS_WDTRESET bit to use watchdog reset

When SYS_WDTRESET is set, watchdog timer reset request
is ignored by power management unit.

Signed-off-by: Jonghwan Choi <jhbird.choi@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>