configs: defconfig for Sipeed Maix in S-mode

Provide a defconfig that can be used to build U-Boot for the Maix boards
running upon OpenSBI.

Update the documentation.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>