mips: update README.mips and NEWS for 3.9

Minor update to README.mips and NEWS section to state that MIPS64 support is
new in 3.9, as well as support for MIPS DSP ASE on MIPS32.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@13661 a5019735-40e9-0310-863c-91ae7b9d1cf9