mfd: 88pm886: Add GPADC cell

Add a cell for the PMIC's onboard General Purpose ADC.

Acked-by: Karel Balej <balejk@matfyz.cz> # for the PMIC
Signed-off-by: Duje Mihanović <duje@dujemihanovic.xyz>
Link: https://lore.kernel.org/r/20250911-88pm886-gpadc-v4-3-60452710d3a0@dujemihanovic.xyz


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