mfd: s2mps11: Add device tree support

This patch adds DT compatible string for s2mps11 and binding documentation.

Reviewed-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>