powerdomain: add pwrdm_get_mem_bank_count()

Add a function pwrdm_get_mem_bank_count() to the powerdomain code to
return the number of memory banks contained inside an OMAP
powerdomain.  This will be used by the PM code to configure the memory
bank retention state.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>