mfd: sec-core: Enable RTC on S2MPS11 PMIC

The S2MPS11 PMIC (present on Arndale Octa board) has the same RTC module
as S2MPS14 device. Add respective mfd cell to enable it.

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