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>