dt-bindings: mfd: mt6397: Add compatibles for MT6331 RTC and keys

The MT6331 PMIC provides many sub modules: regulators, audio
codec, LED controller, keys, RTC and some GPIOs.
It is always paired with a MT6332 Companion PMIC, which provides
thermistors, WLEDs (display LED backlight), secondary AP cluster
regulators, modem clocks, battery charger and fuel gauge.

Add the necessary compatibles to start implementing the basics.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220617110728.90132-2-angelogioacchino.delregno@collabora.com