MIPS: Add SYSCALL to uasm.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/976/


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