qemu_v8: fix error in edk2-clean

'make edk2-clean' fails with the following error:

 build.py...
  : error 000E: One Path in PACKAGES_PATH doesn't exist
         /home/jerome/work/optee_repo_qemu_v8/edk2/../edk2-platforms

The reason is $(ROOT)/edk2-platforms does not exist on this platform.
Fix this by using the proper platform-specific variable
$(EDK2_PLATFORMS_PATH) in the edk2-clean-common target like is done in
edk2-common.

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/common.mk b/common.mk
index d4a172b..dad0539 100644
--- a/common.mk
+++ b/common.mk
@@ -356,7 +356,7 @@
 .PHONY: edk2-clean-common
 edk2-clean-common:
 	$(call edk2-env) && \
-	export PACKAGES_PATH=$(EDK2_PATH):$(ROOT)/edk2-platforms && \
+	export PACKAGES_PATH=$(EDK2_PATH):$(EDK2_PLATFORMS_PATH) && \
 	source $(EDK2_PATH)/edksetup.sh && \
 	$(MAKE) -j1 -C $(EDK2_PATH)/BaseTools clean && \
 	$(call edk2-call) cleanall