feat(drivers/usb): add a USB device stack

Add a new USB framework to manage an USB device profile (USBD)
based on a peripheral controller driver (PCD).

This USB stack can be use to implement any Universal Serial Bus Device
Class in TF-A on top of a USB driver defined in the platform.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Change-Id: I7971ec6d952edec3511157a198e6e5359df4346b
2 files changed