MIPS: BMIPS: restrict DTB selection to BMIPS_GENERIC

Since we are always sourcing arch/mips/bmips/Kconfig and there is no
dependency on BMIPS_GENERIC, we will offer building BMIPS-related DTBs
while this is not relevant for the other MIPS platforms.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: cernekee@gmail.com
Cc: jaedon.shin@gmail.com
Patchwork: https://patchwork.linux-mips.org/patch/9603/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>