command: Use a constant pointer for the help

command: Use a constant pointer for the help

This text should never change during execution, so it makes sense to
use a const char * so that it can be declared as const in the code.
Update struct cmd_tbl with a const char * pointer for 'help'.

We cannot make usage const because of the bmode command, used on mx53ppd
for example.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>