mips: add missing dtb-y to arch/mips/dts/Makefile

Since commit 27cb7300 ("Ensure device tree DTS is compiled"),
build succeeds irrespective of the correctness of Makefile.

In fact, you can compile any defconfig without adding any entry in
arch/*/dts/Makefile.

I am going to revert that commit, so device tree must be explicitly
listed in Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>