regulator: fp9931: Make vin-supply mandatory

Robby Cai <robby.cai@nxp.com> says:

The FP9931 regulator requires a valid "vin" supply to operate correctly.
Therefore, the driver should treat "vin" as a mandatory supply.

This patchset updates the binding documentation to mark vin-supply as a
required property, and modifies the driver accordingly. As suggested in
the reviews from Andreas and Mark, v2 switches to using
devm_regulator_get() since the supply is mandatory.