doc: add DeepComputing FML13V01 documentation

Describe building U-Boot for the board and booting.

Carve out common information for JH7110 boards into an include.

Reviewed-by: E Shattow <e@freeshell.de>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/doc/board/starfive/index.rst b/doc/board/starfive/index.rst
index 2cba1b6..66abc6f 100644
--- a/doc/board/starfive/index.rst
+++ b/doc/board/starfive/index.rst
@@ -6,6 +6,7 @@
 .. toctree::
    :maxdepth: 1
 
+   deepcomputing_fml13v01
    milk-v_mars
    pine64_star64
    visionfive2