[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@309910 91177308-0d34-0410-b5e6-96231b3b80d8