avcodec/mips: MSA (MIPS-SIMD-Arch) optimizations for HEVC idct functions

This patch adds MSA (MIPS-SIMD-Arch) optimizations for HEVC idct functions in new file hevc_idct_msa.c
Adds new generic macros (needed for this patch) in libavutil/mips/generic_macros_msa.h

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