uefi-raw: Add Device Path protocol definitions.

Some Device Paths have variable length trailer data, so a fully safe API
would be rather difficult to provide without reliance on custom DSTs
(and somewhat breaking the seperation of concerns between uefi-raw and uefi).

Presence of this in just uefi-raw is still useful for applications intending
to examine Device Paths.

Signed-off-by: Mate Kukri <[email protected]>

Author: kukrimate