arm64: dts: imx8mq: Mark iomuxc_gpr as i.MX6Q compatible

Mark iomuxc_gpr as compatible with "fsl,imx6q-iomuxc-gpr" in order for
to allow i.MX6 PCIe driver to use it.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Leonard Crestez <leonard.crestez@nxp.com>
Cc: "A.s. Dong" <aisheng.dong@nxp.com>
Cc: Richard Zhu <hongxing.zhu@nxp.com>
Cc: linux-imx@nxp.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Shawn Guo <shawnguo@kernel.org>