[LLDB][MIPS] Fix Floating point Registers Encoding

Patch by Nitesh Jain.

Summary: Currently floating point regsiters has eEncodingUint encoding. Hence register write  '1.25' will failed. This patch add eEncodingIEEE754 encoding for floating point registers( - ). This patch will fix test_fp_register_write in TestRegisters.py

Reviewers: clayborg, sagar
Subscribers: mohit.bhakkad, jaydeep, bhushan, sdardis, lldb-commits
Differential: D18853
llvm-svn: 270208