mgmt: ec_host_cmd: add USB backend

Add a new ec_host_cmd backend based on USB. The backend uses the new USB
device stack.

The backend implements a vendor-specific (Google) USB interface. It uses
three endpoints:
- bulk OUT to transfer a host command request
- bulk IN to transfer a host command response
- interrupt IN to signal an event and readiness of the response

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>