dt-bindings: imx6q-pcie: Add i.MX8MM PCIe EP mode compatible string

Add i.MX8MM PCIe endpoint mode compatible string.

Link: https://lore.kernel.org/r/1673847684-31893-2-git-send-email-hongxing.zhu@nxp.com
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
