dt-bindings: power: fsl,imx-gpc: document fsl,imx6ul-gpc compatible

Document existing fsl,imx6ul-gpc compatible used with fsl,imx6q-gpc
fallback:

  imx6sl-evk.dtb: gpc@20dc000: compatible: ['fsl,imx6sl-gpc', 'fsl,imx6q-gpc'] is too long

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>