reggroups: Add test and docs for `info reg $reggroup` feature

reggroups: Add test and docs for `info reg $reggroup` feature

Until now this feature has existed but was not documented.  Adding docs
and tests.

gdb/ChangeLog:

yyyy-mm-dd  Stafford Horne  <shorne@gmail.com>

	* infcmd.c (_initialize_infcmd): Add help for info reg $reggroup
	and info all-registers $reggroup feature.

gdb/doc/ChangeLog:

yyyy-mm-dd  Stafford Horne  <shorne@gmail.com>

	* gdb.texinfo (Registers): Document info reg $reggroup feature.

gdb/testsuite/ChangeLog:

yyyy-mm-dd  Stafford Horne  <shorne@gmail.com>

	* gdb.base/reggroups.c: New file.
	* gdb.base/reggroups.exp: New file.