next-checks: check cargo doc for all rf-a platforms

`cargo doc` is not a platform independent test in the case
of the rf-a repository..
For the rest of the crates, `cargo doc` should still be a platform
independent test.

 * Run `cargo doc` on rf-a for all platforms. Remove rf-a's
   `cargo doc` tests from the platform independent testing scripts
   and add it to the next-checks (static checks) scripts.

Change-Id: Ib605d674bff2490f84837a1c565dbd333fa5657b
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
3 files changed