mfd: max77693: Add muic of_compatible in mfd_cell

This patch adds muic of_compatible in order to use the muic device
driver in device tree.

Signed-off-by: Maciej Purski <m.purski@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>