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 <festevam@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>