gdb: add some const in gdb/reggroups.c

This commit makes the 'struct reggroup *' argument const for the
following functions:

  reggroup_next
  reggroup_prev
  reggroup_name
  reggroup_type

There are other places that could benefit from const in the
reggroup.{c,h} files, but these will be changing in further commits.

There should be no user visible changes after this commit.