regulator: gpio: Add input_supply support in gpio_regulator_config

regulator: gpio: Add input_supply support in gpio_regulator_config

This is simillar as fixed-regulator.
Used to extract regulator parent from the device tree.

Without that property used, the parent regulator can be shut down (if not an always on).
Thus leading to inappropriate behavior:
On am62-SP-SK this fix is required to avoid tps65219 ldo1 (SDMMC rail) to be shut down after boot completion.

Signed-off-by: Jerome Neanne <jneanne@baylibre.com>
Link: https://lore.kernel.org/r/20220929132526.29427-2-jneanne@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>