[mips] synci microMIPS instruction definition.

Add synci to the microMIPS instruction definitions, mark the MIPS sync & synci
as not being part of microMIPS. This does not cover the sync instruction alias,
as that will be handled with a different patch. Add sync to the valid tests for
microMIPS.

Reviewers: vkalintiris

Differential Revision: https://reviews.llvm.org/D25795

llvm-svn: 284962