MIPS: Add base architecture support for RI and XI.

Originally both Read Inhibit (RI) and Execute Inhibit (XI) were
supported by the TLB only for a SmartMIPS core. The MIPSr3(TM)
Architecture now defines an optional feature to implement these
TLB bits separately. Support for one or both features can be
checked by looking at the Config3.RXI bit.

Signed-off-by: Steven J. Hill <sjhill@mips.com>
Acked-by: David Daney <david.daney@cavium.com>