ARM: OMAP2+: Add minimal support for booting vexpress

With multiplatform support enabled we are now always
building in vexpress. Let's enable few drivers also as
this allows us to boot omap2plus zImage in qemu for
testing multiplaform related changes.

Tested-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>