ARM: dts: imx7s: Mark 'gpr' compatible with i.MX6 variant

List GPR block as compatible "fsl,imx6q-iomuxc-gpr" to support drivers
requesting it that way (PCIe driver is one example).

Cc: yurovsky@gmail.com
Cc: Dong Aisheng <aisheng.dong@nxp.com>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>