mfd: max14577: Add of_compatible to extcon mfd_cell

Add of_compatible ("maxim,max14577-muic") to the mfd_cell for extcon
driver. If entry with such compatible is present in the DTS, the extcon
driver will have of_node set.

This may be useful for extcon consumers and it is documented in
bindings documentation.

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