dt-bindings: ata: imx: Document 'target-supply'

The 'target-supply' property is used by some i.MX devicetree files.

Document it to fix the following dt-schema warning:

'target-supply' does not match any of the regexes: '^pinctrl-[0-9]+$'

Signed-off-by: Fabio Estevam <[e-mail]>
Acked-by: Conor Dooley <[e-mail]>
Signed-off-by: Damien Le Moal <[e-mail]>

Documentation/devicetree/bindings/ata/imx-sata.yaml

···
   power-domains:
     maxItems: 1
 
+  target-supply:
+    description: Power regulator for the SATA target device.
+
 required:
   - compatible
   - reg