oeqa: Create test_20_fvp_devices to reorder tests

Create test_20_fvp_devices which inherits from fvp_devices
in meta-arm, It adds no changes to these tests at all but
it allows putting the tests in the correct order to avoid
restarting the FVP multiple times.

Signed-off-by: Ziad Elhanafy <ziad.elhanafy@arm.com>