mips32: Add support for mips32 DSP instruction set.

Add support for mips32 DSP and DSP revision 2 ASE.
More details about the mips32 DSP(r2) ASE:
http://www.mips.com/media/files/MD00566-2B-MIPSDSP-QRC-01.00.pdf
Applied patch provided by Maja Gagic <maja.gagic@rt-rk.com>


git-svn-id: svn://svn.valgrind.org/vex/trunk@2732 8f6e269a-dfd6-0310-a8e1-e2731360e62c