omap3: clock: Fixed dpll3_m2x2 rate calculation

Current calculation does not take into account any changes to M2 divisor, and
thus when we change VDD2 OPP, dpll3_m2x2 rate does not change. Fixed by
re-routing dpll3_m2x2 parent to dpll3_m2.

Signed-off-by: Tero Kristo <tero.kristo@nokia.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>