gdb: Add 'tui reg prev' command.

gdb: Add 'tui reg prev' command.

There is already a 'tui reg next' command, this adds a symmetric 'tui
reg prev' command.

gdb/ChangeLog:

	* tui/tui-regs.c (tui_reg_prev_command): New function.
	(_initialize_tui_regs): Add 'prev' command for 'tui reg'.
	* reggroups.c (reggroup_prev): New function.
	* reggroups.h (reggroup_prev): Add declaration.  Update comment.

gdb/doc/ChangeLog:

	* gdb.texinfo (TUI Commands): Add 'tui reg prev' details.