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>
diff --git a/doc/board/sophgo/index.rst b/doc/board/sophgo/index.rst
index e097afd..26dba4a 100644
--- a/doc/board/sophgo/index.rst
+++ b/doc/board/sophgo/index.rst
@@ -6,3 +6,4 @@
    :maxdepth: 1
 
    milkv_duo
+   licheerv_nano