dt-bindings: power: fsl,imx-gpc: Document fsl,imx6sll-gpc

Document the existing fsl,imx6sll-gpc compatible used with fsl,imx6q-gpc
fallback.

This fixes the following dt-schema warning:

['fsl,imx6sll-gpc', 'fsl,imx6q-gpc'] is too long

Signed-off-by: Fabio Estevam <[e-mail]>
Acked-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

Documentation/devicetree/bindings/power/fsl,imx-gpc.yaml

···
           - enum:
               - fsl,imx6qp-gpc
               - fsl,imx6sl-gpc
+              - fsl,imx6sll-gpc
               - fsl,imx6sx-gpc
               - fsl,imx6ul-gpc
           - const: fsl,imx6q-gpc