regulator: lp8788-buck: Fully convert to GPIO descriptors

regulator: lp8788-buck: Fully convert to GPIO descriptors

This converts the LP8788 BUCK regulator driver to use GPIO
descriptors.

BUCK1 can use one DVS GPIO and BUCK2 can use two DVS GPIOS,
and no more so just hardcode two GPIO descriptors into
the per-DVS state containers.

Obtain the descriptors from each regulators subdevice.

As there are no in-tree users, board files need to populate
descriptor tables for the buck regulator devices when
they want to use this driver. BUCK1 need a GPIO descriptor
at index 0 and BUCK2 needs two GPIO descriptors at
indices 0 and 1.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://msgid.link/r/20240220-descriptors-regulators-v1-3-097f608694be@linaro.org
Acked-by: Lee Jones <lee@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>