ovmf-shell-image: image is only buildable on x86-64

This image is only buildable for x86-64, so add a COMPATIBLE assignment
to ensure it isn't attempted on others.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit bdd82086)
Signed-off-by: Steve Sakoman <steve@sakoman.com>