ARM: OMAP: DRA7: powerdomain data: Remove unused pwrsts_mem_ret

As per the latest revision F of public TRM for DRA7/AM57xx SoCs
SPRUHZ6F[1] (April 2016), with the exception of MPU power domain, all
other power domains do not have memories capable of retention since
they all operate in either "ON" or "OFF" mode. For these power states,
the retention state for memories are basically ignored by PRCM and does
not require to be programmed.

[1] http://www.ti.com/lit/pdf/spruhz6

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>