dt-bindings: imx6q-pcie: Add iMX8MM PCIe compatible string

Add the i.MX8MM PCIe compatible string.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1646293805-18248-1-git-send-email-hongxing.zhu@nxp.com