Documentation: mfd: s2mps11: Describe S5M8767 and S2MPS14 clocks

Add bindings documentation for clocks on S5M8767 and S2MPS14 devices.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>