usb: Add base USB MCTP definitions

Upcoming changes will add a USB host (and later gadget) driver for the
MCTP-over-USB protocol. Add a header that provides common definitions
for protocol support: the packet header format and a few framing
definitions. Add a define for the MCTP class code, as per
https://usb.org/defined-class-codes

.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/20250221-dev-mctp-usb-v3-1-3353030fe9cc@codeconstruct.com.au


Signed-off-by: Jakub Kicinski <kuba@kernel.org>