dts: bindings: Add generic Xiao interconnect

* With existence of Adafruit Qt Py boards, and upcoming wireless
  Xiao from Seeeduino, define nexus node and peripheral node
  labels for use with shields that accept any Xiao format board.
* Adds `&xiao_d`, `&xiao_spi`, `&xiao_i2c` and `&xiao_serial` generic
  node labels.
* Add new 'seeed-xioa-header.yaml` to document new nexus node.

Signed-off-by: Peter Johanson <peter@peterjohanson.com>