overlays: Add vl805 overlay

With the automatic VL805 support being removed from the standard
CM4 dtb (since most CM4 carriers don't have a VL805), retain support
on those that do by creating a "vl805" overlay that restores the
deleted "usb@0,0" node.

The "vl805" overlay will be loaded automatically (after an upcoming
firmware update) on CM4 boards where the EEPROM config includes the
setting VL805=1.

See: https://forums.raspberrypi.com/viewtopic.php?t=326088



Signed-off-by: Phil Elwell <phil@raspberrypi.com>