regulator: lp8788-ldo: Pass descriptor instead of GPIO number

Instead of passing a global GPIO number, pass a descriptor looked
up with the standard devm_gpiod_get_index_optional() call.

This driver has supported passing a LDO enable GPIO for years,
yet this facility has never been put to use in the upstream kernel.
If someone desires to put in place GPIO control for the LDOs,
this can be done by adding a GPIO descriptor table in the MFD
nexus in drivers/mfd/lp8788.c for the LDO device when spawning the
MFD children, or using a board file.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>