tdesc: handle arbitrary strings in tdesc_register_in_reggroup_p

tdesc: handle arbitrary strings in tdesc_register_in_reggroup_p

tdesc_register_in_reggroup_p in now able to handle arbitrary
groups. This is useful when groups are created while the
target descriptor file is received from the remote.

This can be the case of a soft core target processor where
registers/groups can change.

gdb/ChangeLog:

yyyy-mm-dd  Franck Jullien  <franck.jullien@gmail.com>
	    Stafford Horne  <shorne@gmail.com>

	* target-descriptions.c (tdesc_register_in_reggroup_p): Support
	arbitrary strings.
	(tdesc_use_registers): Add logic to register new reggroups.
	(tdesc_reg::group): Update comment to indicate we allow
	arbitrary strings.
	* NEWS (Changes since GDB 8.0): Announce that GDB supports
	arbitrary reggroups.

gdb/testsuite/ChangeLog:

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

	* gdb.xml/extra-regs.xml: Add example foo reggroup.
	* gdb.xml/tdesc-regs.exp: Add test to check for foo reggroup.

gdb/doc/ChangeLog:

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

	* gdb.texinfo (Target Description Format): Explain that arbitrary
	strings are now allowed for register groups.