OMAP2 clockdomain: add virt_opp_clkdm

Every OMAP2/3 clock will need to be associated with a clockdomain.
However, the "virt_prcm_set" clock has no clockdomain, since it is a
virtual clock without any hardware referents.  So, create a new
clockdomain, "virt_clkdm", for this clock.  This clockdomain should be
reusable for OMAP3 virtual clock nodes.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>