Makefile: fix make clean to remove platform artifacts

Update the `clean` target to correctly remove generated `.xml` and `.bin`
files from nested subdirectories under `platforms/*/[emmc|ufs]` paths.

Signed-off-by: Viswanath Kraleti <[email protected]>

Author: vkraleti

Makefile

@@ -40,4 +40,4 @@ install: $(BINS)
 	install -m 755 $^ $(DESTDIR)$(PREFIX)/bin
 
 clean:
-	@rm -f platforms/*/*.xml platforms/*/*.bin
+	@rm -f platforms/*/*/*.xml platforms/*/*/*.bin