Add SPDX license identifier to Arm files ...

Add license identifier to files which Arm owns the copyright to,
and contain either BSD-3 or Apache-2.0 licenses. This is to address
license errors raised by scancode analysis.

Relocate USB target specific code to root `targets` directory ...

All target specific source and header files should be in the `targets`
directory located at the root of the Mbed OS repository.