[mips] Add new format for dmtc2/dmfc2 for Octeon CPUs.

[mips] Add new format for dmtc2/dmfc2 for Octeon CPUs.

Octeon CPUs use dmtc2 rt,imm16 and dmfcp2 rt,imm16 for the crypto coprocessor.
E.g. dmtc2 rt,0x4057 starts calculation of sha-1.

I had to introduce a new deconding namespace to avoid a decoding conflict.

Reviewed By: dsanders

Differential Revision: http://reviews.llvm.org/D10083

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@238439 91177308-0d34-0410-b5e6-96231b3b80d8