The Lichee Pi Zero board has a "dock board" which needs to be soldered
with the 1.27mm stamp holes on a Lichee Pi Zero board.

It features:
- Onboard MIC and headphone jack (not supported yet)
- Ethernet port (not supported yet)
- An extra MicroSD slot connected to MMC1 controller
- four keys connected to the LRADC.

As it needs to be soldered with the main board to use, add a stand-alone
device tree for it.
Signed-off-by: NIcenowy Zheng <icenowy@aosc.io>
Signed-off-by: NMaxime Ripard <maxime.ripard@free-electrons.com>