OMAP2/3 PM: Adding powerdomain APIs for reading the next logic and mem state

This patch adds APIs pwrdm_read_logic_retst and
pwrdm_read_mem_retst for reading the next programmed
logic and memory state a powerdomain is to hit in event
of the next power domain state being retention.
These are needed for OSWR support.

Signed-off-by: Thara Gopinath <thara@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>