hw/hexagon: Add globalreg model

Some of the system registers are shared among all threads
in the core.  This object contains the representation and
interface to the system registers.

Reviewed-by: Sid Manning <sid.manning@oss.qualcomm.com>
Acked-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Brian Cain <brian.cain@oss.qualcomm.com>