bindings: mfd: s2mps11: Add documentation for s2mps15 PMIC

Add dt-binding documentation for s2mps15 PMIC device. The s2mps15 device
is similar to s2mps11/14 PMIC device and has 27 LDO and 10 buck regulators.
This also supports RTC and three 32.768KHz clock outputs.

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>
Signed-off-by: Lee Jones <lee.jones@linaro.org>