arm64: dts: khadas-vim3: add commented support for PCIe

arm64: dts: khadas-vim3: add commented support for PCIe

The VIM3 on-board  MCU can mux the PCIe/USB3.0 shared differential
lines using a FUSB340TMX USB 3.1 SuperSpeed Data Switch between
an USB3.0 Type A connector and a M.2 Key M slot.
The PHY driving these differential lines is shared between
the USB3.0 controller and the PCIe Controller, thus only
a single controller can use it.

The needed DT configuration when the MCU is configured to mux
the PCIe/USB3.0 differential lines to the M.2 Key M slot is
added commented and may be uncommented to disable USB3.0 from the
USB Complex and enable the PCIe controller.

The End User is not expected to uncomment the following except for
testing purposes, but instead rely on the firmware/bootloader to
update these nodes accordingly if PCIe mode is selected by the MCU.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Andrew Murray <andrew.murray@arm.com>