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>