[RISCV] Remove include of RISCVRegisterInfo.h from RISCVBaseInfo.h

RISCVRegisterInfo.h is part of the CodeGen layer. The Utils library
is intended to be shared with the MC layer so shouldn't use files
from the CodeGen layer.

The register enum names are already available from
RISCVMCTargetDesc.h. It appears what was coming from this include
was a transitive include of the Register class which I've replaced
with MCRegister. Register has a constructor from MCRegister so it
should be convertible.