devicetree: mfd: max14577: Add device tree bindings document

Add document describing device tree bindings for MAX14577 MFD
drivers: MFD core, extcon, regulator and charger.

Both MAX14577 and MAX77836 chipsets are documented.

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