MIPS: Add build support for the MIPS R6 ISA

Add build support for the latest revision (R6) of the MIPS ISA.
microMIPS is not yet supported.

Link: http://www.linux-mips.org/archives/linux-mips/2015-01/msg00386.html
Cc: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>