board: amlogic: vim3: add support for dynamic PCIe enable

board: amlogic: vim3: add support for dynamic PCIe enable

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.

This adds this dynamic switching right before booting Linux
and the configuration steps in the boards documentation.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
[narmstrong: fixed warning by replacing min() by min_t()]