[ARM] Tidy up banked registers encoding

[ARM] Tidy up banked registers encoding

Moves encoding (SYSm) information of banked registers to ARMSystemRegister.td,
where it rightly belongs and forms a single point of reference in the code.

Reviewed by: @fhahn, @rovka, @olista01
Differential Revision: https://reviews.llvm.org/D36219

llvm-svn: 309910