mfd: sec-core: Select different RTC regmaps for devices

This patch prepares for adding support for S2MPS14 RTC driver by
selecting different regmaps for S2MPS1X/S5M876X RTC devices.

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