qemu_v8.mk: update edk2-call to invoke the EDK2 build script

Like other platforms (fvp.mk, hikey.mk, hikey960.mk, hikey_debian.mk)
define the edk2-call macro to invoke the EDK2 build script rather than
$(MAKE) directly. Indeed, the edk2-clean-common target in common.mk
expects $(edk2-call) to take the 'cleanall' argument, which the EDK2
build script does accept, but the EDK2 makefile doesn't (there is no
cleanall rule in EDK2).

Fixes:

 $ make clean
 [...]
 make[1]: Entering directory '/home/jerome/optee_repo_qemu_v8/edk2'
 make[1]: *** No rule to make target 'cleanall'.  Stop.

Another option to fix the above error would be to change the edk2-clean
target to no use edk2-clean-common and call "$(MAKE) clean" directly.
But then, it would be inconsistent with other platforms and the EDK2
build script may be a better interface than raw make (more stable).

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reported-by: Joel Anderson <jeander@vt.edu>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>