dt-bindings: imx-gpc: add i.MX6 QuadPlus compatible

While the GPC on i.MX6QP is mostly comptible to the i.MX6Q one,
the QuadPlus requires special workarounds for hardware erratum
ERR009619.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

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

@@ -5,7 +5,10 @@ The i.MX6 General Power Control (GPC) block contains DVFS load tracking
 counters and Power Gating Control (PGC).
 
 Required properties:
-- compatible: Should be "fsl,imx6q-gpc" or "fsl,imx6sl-gpc"
+- compatible: Should be one of the following:
+  - fsl,imx6q-gpc
+  - fsl,imx6qp-gpc
+  - fsl,imx6sl-gpc
 - reg: should be register base and length as documented in the
   datasheet
 - interrupts: Should contain one interrupt specifier for the GPC interrupt