dt-bindings: gpio-regulator: add vin-supply property support

This is same scheme as fixed-regulator.

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/20220929132526.29427-3-jneanne@baylibre.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/regulator/gpio-regulator.yaml

@@ -85,6 +85,9 @@ properties:
       - current
     default: voltage
 
+  vin-supply:
+    description: Input supply phandle.
+
 required:
   - compatible
   - regulator-name