ARM: OMAP2+: Add custom prwdm_operations for 81xx to support dm814x

Looking at the TI kernel tree I noticed that dm81xx need custom
ti81xx_pwrdm_operations. Let's also change dm816x over to use them
as the registers are different for dm81xx compared to others.

Cc: Matthijs van Duin <matthijsvanduin@gmail.com>
Cc: Paul Walmsley <paul@pwsan.com>
Cc: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>