MIPS: dts: Always descend vendor subdirectories

MIPS: dts: Always descend vendor subdirectories

Commit 41528ba6afe6 ("MIPS: DTS: Only build subdir of current platform")
broke building of all DTBs when CONFIG_OF_ALL_DTBS is enabled unless all
the various kconfig options were also enabled. The only effect that commit
had was getting rid of some harmless build lines such as:

      AR      arch/mips/boot/dts/mti/built-in.a

Those lines were part of the built-in DTB support. Since commit
04e4ec98e405 ("MIPS: migrate to generic rule for built-in DTBs"), how the
built-in DTBs are handled has changed and those lines are no longer
generated, so revert to the prior behavior.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>