mfd: sec: Add support for S2MPS15 PMIC

Add support for S2MPS15 PMIC which is similar to S2MPS11 PMIC. The S2MPS15
PMIC supports 27 LDO regulators, 10 buck regulators, RTC, three 32.768KHz
clock outputs and battery charger. This patch adds initial support for
LDO and buck regulators of S2MPS15 device.

Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
[Alim: Added s2mps15_devs like rtc and clk and related changes]
Signed-off-by: Lee Jones <lee.jones@linaro.org>