cmd: add sdp command

Add a new command to start USB Serial Download Protocol (SDP)
state machine.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>
Reviewed-by: Ɓukasz Majewski <lukma@denx.de>
Reviewed-by: Stefano Babic <sbabic@denx.de>
3 files changed