ARM: OMAP: Add GPMC support for OMAP2

Implement basic support for General-Purpose Memory Controller
as found on OMAP2420.

Dynamic CS address space allocation still needs to be done.

Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>