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.

(From OE-Core rev: bdd82086)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit db87aab8e219d2520dcd8d15da89110aadf3d41e)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>