arm64: dts: imx8mm-evk: Add the pcie support on imx8mm evk board

Add the PCIe support on iMX8MM EVK boards.
And set the default reference clock mode.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Tim Harvey <tharvey@gateworks.com>
Tested-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>