Images: Build s390x controller. (kubernetes#12125)

Author: Gacko

Diff for: Makefile

@@ -240,8 +240,8 @@ ensure-buildx:
 show-version:
 	echo -n $(TAG)
 
-PLATFORMS ?= amd64 arm arm64
-BUILDX_PLATFORMS ?= linux/amd64,linux/arm,linux/arm64
+PLATFORMS ?= amd64 arm arm64 s390x
+BUILDX_PLATFORMS ?= linux/amd64,linux/arm,linux/arm64,linux/s390x
 
 .PHONY: release # Build a multi-arch docker image
 release: ensure-buildx clean