ARM: OMAP3: Rename omap3.h to omap.h to be generic as all SoCs

This is in preperation of using generic cross OMAP code.

Signed-off-by: Nishanth Menon <nm@ti.com>
Tested-by: Matt Porter <mporter@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>