MLK-24171-1 arm64: dts: imx8mp: verify the pcie pll sys ref clock

MLK-24171-1 arm64: dts: imx8mp: verify the pcie pll sys ref clock

Verify the PCIe PLL_SYS reference clock source on EVK board.
The external OSC clock is used as PCIe REF clock source in default.
NOTE: Change the ext_osc of pcie/pcie_phy to '0' when enable SYS_PLL
clock mode.

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Reviewed-by: Fugang Duan <fugang.duan@nxp.com>