MIPS: kernel: cpu-probe: Detect unique RI/XI exceptions

MIPS: kernel: cpu-probe: Detect unique RI/XI exceptions

Detect if the core supports unique exception codes for the
Read-Inhibit and Execute-Inhibit exceptions and set the
option accordingly. The RI/XI exception support is detected
by setting the 27th bit (IEC) of the PageGrain C0 register
and reading back the value of that register to verify the
bit is enabled.

Signed-off-by: Leonid Yegoshin <[email protected]>
Signed-off-by: Markos Chandras <[email protected]>
Cc: [email protected]
Patchwork: https://patchwork.linux-mips.org/patch/7340/
Signed-off-by: Ralf Baechle <[email protected]>