mfd: max8997: Add of_compatible to Extcon and Charger mfd_cell

Add of_compatible ("maxim,max8997-muic") to the mfd_cell to have a
of_node set in the extcon driver.

Add of_compatible ("maxim,max8997-battery") to the mfd_cell to configure
the charger driver.

Signed-off-by: Timon Baetz <timon.baetz@protonmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>