tfa-next(next-checks-clippy): use rust Makefile to list platforms

tfa-next has added a new `list_platforms` target for the
rust/Makefile. Use this target instead of hardcoding the available
platforms.

Change-Id: I5701e366b5cf4f3ba8a8f5d58a02ed08237cc353
Signed-off-by: Tomás González <tomasagustin.gonzalezorlando@arm.com>
1 file changed