ALSA: dummy - Support high-res timer mode

Allow snd-dummy driver to use high-res timer as its timing source
instead of the system timer.  The new module option "hrtimer" is added
to turn on/off the high-res timer support.  It can be switched even
dynamically via sysfs.
Signed-off-by: Takashi Iwai <tiwai@suse.de>