MIPS: Introduce API for enabling & disabling L2 prefetch

Introduce new functions in struct bcache_ops to enable & disable L2
cache prefetching, and to retrieve the current state of L2 prefetching.
This will be used in later patches.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11179/


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