mfd: s2mps11: Add binding documentation for Samsung S2MPS13 PMIC

This patch adds the binding documentation for Samsung S2MPS13 PMIC
which is similiar with existing S2MPS14 PMIC. S2MPS13 has the different number
of regulators from S2MPS14 and RTC/Clock is the same with the S2MPS14.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Sangbeom Kim <sbkim73@samsung.com>
Reviewed-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>