dt-bindings: power: fsl,imx-gpc: correct compatibles

fsl,imx6qp-gpc, fsl,imx6sl-gpc and fsl,imx6sx-gpc use generic fallback
fsl,imx6q-gpc in the DTS, even though these are different devices.  The
common part (thus the fallback) is a interrupt controller routing back
to GIC.

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