cmd: introduce 'write' command

cmd: introduce 'write' command

It's almost no extra code to hook up a buddy to the 'read' command. In
fact, since the command is passed its own 'struct cmd_tbl', we can use
the exact same callback, and let it figure out for itself whether it
was invoked as "read" or "write".

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>