ARM: OMAP2+: gpmc: Low power transition support

GPMC is hardware controller for external memory interfaces.
This patch adds suspend/resume support for GPMC driver.
It also preserves GPMC register configurations across device low-power states
in which GPMC hardware can be powered-off.
gpmc_suspend()/gpmc_resume() are called by default by core PM framework as part
of driver's runtime PM callbacks.

Signed-off-by: Philip Avinash <avinashphilip@ti.com>
Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>