MIPS: Enable IOREMAP_PROT config option for MIPS cpus

Allows the users of ptrace to access memory mapped by the ptraced process
using the same cache coherency attributes as the original process.
For example while using gdb with ioremap_prot() incorporated, both gdb and
the process being traced will have same cache coherency attributes.

Signed-off-by: Hassan Naveed <hnaveed@wavecomp.com>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Patchwork: https://patchwork.linux-mips.org/patch/20955/
Cc: <linux-mips@linux-mips.org>