dt-bindings: mfd: Add entry for Marvell 88PM886 PMIC

Marvell 88PM886 is a PMIC with several subdevices such as onkey,
regulators or battery and charger. It comes in at least two revisions,
A0 and A1 -- only A1 is described here at the moment.

Signed-off-by: Karel Balej <balejk@matfyz.cz>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20240531175109.15599-2-balejk@matfyz.cz


Signed-off-by: Lee Jones <lee@kernel.org>