mt6359: Improve device tree integration for codec

This merge request follows up on a previous attempt to fix a device tree warning by adding the #sound-dai-cells property.

Previously, the device tree referenced the DAIs by pointing to the PMIC node instead of the codec subnode. To reference the codec subnode properly in the device tree, we need to add a compatible string to the MFD driver. This ensures the codec device gets its own of_node assigned.