MLK-24171-2 dt-binding: phy: update the clock modes of pcie phy

Update the clock modes of iMX8MP PCIe PHY in binding DOC.

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

Documentation/devicetree/bindings/phy/fsl,imx-pcie-phy.txt

@@ -6,6 +6,8 @@ Required properties:
 - reg:		The base address and length of the registers
 - clocks:	Phandles to the clocks for each clock listed in clock-names
 - clock-names:	Must contain "phy"
+- ext_osc:	Specify the reference clock source. 1: external oscilltor is
+  used as PCIe reference clock. 0: internal PLL is used.
 - power-domains: Phandle to the power domain that the device is part of
 
 Example: