regulator: mt6397: Add OF match table

The Documentation/devicetree/bindings/regulator/mt6397-regulator.txt doc
mentions that a compatible "mediatek,mt6397-regulator" is required for
the MT6397 device node but the driver doesn't provide a OF match table so
the platform bus .match fallbacks to match using the driver name instead.

This also means that module auto-loading is broken for this driver since
the MFD driver that register the device, has a .of_compatible set so the
platform .uevent callback reports a OF modalias that's not in the module.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>