clocksource: arch_timer: make virtual counter access configurable

Add a config option to support virtual counter access from userspace.

Change-Id: Ibdb1fd768b748002b90bfc165612c12c8311f8a2
Signed-off-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Venkata Narendra Kumar Gutta <vnkgutta@codeaurora.org>
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Cong Zhang <congzhan@codeaurora.org>