rtc: s5m: Remove unused watchdog and sudden momentary power loss

The WTSR (Watchdog Timer Software Reset) and SMPL (Sudden Momentary
Power Loss) are never enabled. These are left-overs from board files.
After removing them the driver's shutdown callback is empty so get rid
of it as well.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>