xilinx: zynq: Enable mkeficapsule tools compilation

Zynq can use efi capsule infrastructure that's why enable it by default.
For capsule generation for zynq you can use:

pushd spl
../tools/mkeficapsule -g "1ba29a15-9969-40aa-b424-e86121618664" boot.bin \
--index 1 ../capsule1.bin
popd
./tools/mkeficapsule -g "1a5178f0-87d3-4f36-ac63-3b31a23be305" u-boot.img \
--index 2 capsule2.bin

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/a8194ecfa7932f2d8ada5ee508b2a026c782f15e.1660055571.git.michal.simek@amd.com