memory: gpmc: Move omap gpmc code to live under drivers

Just move to drivers as further clean-up can now happen there
finally.

Let's also add Roger and me to the MAINTAINERS so we get
notified for any patches related to GPMC.

Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>