uboot-phytec

doc: board: Add AndesTech ax25-ae350 board doc

This converts README.AX25 and README.ae350 plain text documentation
to reST format, merges them into one ax25-ae350 doc, and adds it to
Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>