aarch64: Rework parse_typed_reg interface

aarch64: Rework parse_typed_reg interface

parse_typed_reg returned a register number and passed the
register type back using a pointer parameter.  It seems simpler
to return the register entry instead, since that has both pieces
of information in one place.

The patch also replaces the boolean in_reg_list parameter with
a mask of flags.  This hopefully makes calls easier to read
(more self-documenting than "true" or "false"), but more
importantly, it allows a later patch to add a second flag.