dt-bindings: imx6q-pcie: Restruct i.MX PCIe schema

Restruct i.MX PCIe schema, derive the common properties, thus they can
be shared by both the RC and Endpoint schema.

Update the description of fsl,imx6q-pcie.yaml, and move the EP mode
compatible to fsl,imx6q-pcie-ep.yaml.

Add support for i.MX8M PCIe Endpoint modes, and update the MAINTAINER
accordingly.

Link: https://lore.kernel.org/r/1676441915-1394-2-git-send-email-hongxing.zhu@nxp.com


Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>