boards: wch: add the ch32v006evt

The WCH CH32V006EVT is an evaluation board for the RISC-V based
CH32V006K8U6 SOC.

The board is equipped with a power LED, reset button, USB port for
power, and two user LEDs.

Add the board definition, documentation, and sample overlay.

Signed-off-by: Michael Hope <michaelh@juju.nz>