OMAP3+: introduce generic ABB support

Adaptive Body Biasing (ABB) modulates transistor bias voltages
dynamically in order to optimize switching speed versus leakage.
Adaptive Body-Bias ldos are present for some voltage domains
starting with OMAP3630. There are three modes of operation:

* Bypass - the default, it just follows the vdd voltage
* Foward Body-Bias - applies voltage bias to increase transistor
  performance at the cost of power.  Used to operate safely at high
  OPPs.
* Reverse Body-Bias - applies voltage bias to decrease leakage and
  save power.  Used to save power at lower OPPs.

Signed-off-by: Andrii Tseglytskyi <andrii.tseglytskyi@ti.com>
Acked-by: Nishanth Menon <nm@ti.com>