mfd: sec: Add clock cell for s2mps11

This patch adds clock to list of mfd cells for s2mps11 and DT documentation
for clock part.

Reviewed-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>