ARM: dts: Add support for more than 2GB of memory for omap5

Some omap5 variants have more than 2GB of memory available as
optional models. Let's update the dts files to use two address
cells similar to what dra7 is using with commit dae320ec


("ARM: dts: DRA7: change address-cells and size-cells").

Acked-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>