regulator: s2mps11: Add support for S2MPS15 regulators

The S2MPS15 PMIC is similar in functionality to S2MPS11/14 PMIC. It contains
27 LDO and 10 Buck regulators and allows programming these regulators via a
I2C interface. This patch adds initial support for LDO/Buck regulators of
S2MPS15 PMIC.

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>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>