mfd: db8500 OPP and sleep handling update

This updates the operating point handling code by:

- Supporting the DDR OPP retention state.
- Supporting another low operating point named
  APE_50_PARTLY_25_OPP
- Adding an interface to figure out if the sleep state change
  was properly achieved.

Signed-off-by: Shreshtha Kumar Sahu <shreshthakumar.sahu@stericsson.com>
Signed-off-by: Rabin Vincent <rabin.vincent@stericsson.com>
Signed-off-by: Mattias Nilsson <mattias.i.nilsson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>