MIPS: Wire up io_pgetevents syscall

Wire up the io_pgetevents syscall that was introduced by commit
7a074e96 ("aio: implement io_pgetevents").

Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/19593/
Cc: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org