MIPS: migrate to generic rule for built-in DTBs

Commit 654102df ("kbuild: add generic support for built-in boot
DTBs") introduced generic support for built-in DTBs.

Select GENERIC_BUILTIN_DTB and BUILTIN_DTB_ALL when the built-in DTB
support is enabled.

DTBs compiled under arch/mips/boot/dts/ will be wrapped by the generic
rule in scripts/Makefile.vmlinux.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>