[mips][ias] Removed DSP/DSPr2 instructions from base architecture valid-xfail.s's.

Summary:
valid-xfail.s is for instructions that should be valid in the given ISA but
incorrectly fail. DSP/DSPr2 instructions are correct to fail since DSP/DSPr2 is
not enabled.

Reviewers: vkalintiris

Subscribers: dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D15072

llvm-svn: 254911