Documentation: mfd: s2mps11: Document support for S2MPS14

Add bindings documentation for S2MPS14 device to the s2mps11 driver.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Tomasz Figa <t.figa@samsung.com>
Cc: devicetree@vger.kernel.org
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Acked-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>