MIPS: Convert BUG() to use unreachable()

Use the new unreachable() macro instead of while(1);

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
CC: linux-mips@linux-mips.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>