mips: boot: use 'targets' instead of extra-y in Makefile

vmlinux.bin.* files are built as prerequisites of other objects.
There is no need to use extra-y, which is planned for deprecation.

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