MIPS: Declare uasm bbit0 and bbit1 functions.

these are already defined, but declaring them allow them to be used
outside of uasm.c.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1872/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>