MIPS: Wire up sched_setattr/sched_getattr syscalls

Wire up for MIPS the new sched_setattr and sched_getattr system calls
added in commit d50dde5a10f3 (sched: Add new scheduler syscalls to
support an extended scheduling parameters ABI) merged in v3.14-rc1.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6502/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>