PCI: imx6: Add i.MX8MP PCIe support

PCI: imx6: Add i.MX8MP PCIe support

Add i.MX8MP PCIe support.
To avoid codes duplication when find the syscon regmap, add the iomux
gpr syscon compatible into drvdata.

Link: https://lore.kernel.org/r/1662109086-15881-8-git-send-email-hongxing.zhu@nxp.com
Tested-by: Marek Vasut <marex@denx.de>
Tested-by: Richard Leitner <richard.leitner@skidata.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>