ARM: OMAP2+: Fix power domain operations regression caused by 81xx

I managed to mess up omap3 power domain operations with commit
7c80a3f8 ("ARM: OMAP2+: Add custom prwdm_operations for 81xx
to support dm814x"), by default we should keep on using the
omap3_pwrdm_operations, only 81xx needs custom handling.

This causes omap3 PM to break so we won't hit off mode any longer
causing idle power consumption go up from less than 10mW to over
50 mW.

Fixs: 7c80a3f8 ("ARM: OMAP2+: Add custom prwdm_operations for
81xx to support dm814x")
Signed-off-by: Tony Lindgren <tony@atomide.com>