mfd: mt6397: Add basic support for MT6331+MT6332 PMIC

Add support for the MT6331 PMIC with MT6332 Companion PMIC, found
in MT6795 Helio X10 smartphone platforms.

This combo has support for multiple devices but, for a start,
only the following have been implemented:
- Regulators (two instances, one in MT6331, one in MT6332)
- RTC (MT6331)
- Keys (MT6331)
- Interrupts (MT6331 also dispatches MT6332's interrupts)

There's more to be implemented, especially for MT6332, which
will come at a later stage.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220627123954.64299-1-angelogioacchino.delregno@collabora.com