MIPS: Repair Kbuild make clean breakage.

When running make clean, Kbuild doesn't process the .config file, so nothing
generates a platform-y variable.  We can get it to descend into the platform
directories by setting $(obj-).

The dec Platform file was unconditionally setting platform-, obliterating
its previous contents and preventing some directories from being cleaned.
This is change to an append operation '+=' to allow cavium-octeon to be
cleaned.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Patchwork: https://patchwork.linux-mips.org/patch/1718/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kbuild

@@ -7,6 +7,10 @@ subdir-ccflags-y := -Werror
 include arch/mips/Kbuild.platforms
 obj-y := $(platform-y)
 
+# make clean traverses $(obj-) without having included .config, so
+# everything ends up here
+obj- := $(platform-)
+
 # mips object files
 # The object files are linked as core-y files would be linked