refactor: FFA_VERSION related refactors

* Use an enum for FF-A versions.
* Replace `MAKE_FFA_VERSION` macro with `make_ffa_version` function.
* Add `ffa_version_is_valid` helper function.
* Add `ffa_version_get_major` and `ffa_version_get_minor` helper
  functions.

Change-Id: Icbc4b1fa16e826c2ec541add65aa7c14fb397f95
Signed-off-by: Karl Meakin <karl.meakin@arm.com>
6 files changed