MIPS: io.h: Define `ioremap_cache'

Signed-off-by: Maciej W. Rozycki <macro@imgtec.com>
Cc: Brian Norris <computersforpeace@gmail.com>
Cc: Rafał Miłecki <zajec5@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12040/


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