Revert "mfd: max8997: Add of_compatible to Extcon and Charger mfd_cell"

This reverts commit 9c03008d.

commit 41a8a027

 ("regulator: dt-bindings: Document charger-supply
for max8997") introduced a binding which uses a property of the max8997
pmic node to configure charger supply, making subnodes for MFD cells
obsolete.

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