doc: add LicheeRV Nano and SG2002 SoC

Provide a page describing the usage of U-Boot on the LicheeRV Nano and a
description of the board.

Signed-off-by: Thomas Bonnefille <thomas.bonnefille@bootlin.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
2 files changed