mfd: sec-core: Fix uninitialized 'regmap_rtc' on S2MPA01

Initialize the 'regmap_rtc' on S2MPA01 to some sane value. Sane at least
for S5M87X chipsets, not S2MPS/S2MPA but it won't be used because
rtc-s5m driver does not support S2MPA01.

This fixes following error:
drivers/mfd/sec-core.c:342:45: warning: ‘regmap_rtc’ may be used uninitialized in this function [-Wuninitialized]

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