drivers: xds110: Clean up command syntax and documentation

drivers: xds110: Clean up command syntax and documentation

Arrange all commands under a top level xds110 command. Fix
documentation to properly reflect the current functionality.

Also updated the links in the document to the new permanent
links for the XDS110 only support.

Patch updated for comments from code review. Return
ERROR_COMMAND_SYNTAX_ERROR for wrong number of args in
commands. Added deprecated commands to src/jtag/startup.tcl.

Change-Id: Ica45f65e1fdf7fa72866f4e28c4f6bce428d8ac9
Signed-off-by: Edward Fewell <efewell@ti.com>
Reviewed-on: http://openocd.zylin.com/5495
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>