ARM: dts: imx6qp: add specific compatible for GPC

Add the more specific QuadPlus compatible to the GPC node, to trigger the
required workarounds in the power domain code.

In regard to the interrupt mapping the QuadPlus controller is fully
compatible to the Quad one, so keep that compatible in place.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>