configure: add support for mips64r6 and i6400 cpu

This is a preparation patch to submit optimized code for MSA (MIPS-SIMD-Architecture)

Signed-off-by: Shivraj Patil <shivraj.patil@imgtec.com>
Reviewed-by: Nedeljko Babic <Nedeljko.Babic@imgtec.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>