omap2/3/4: Clean up entry-macro.s for adding support for omap4 multiboot

Move defines around and set up handlers based on MULTI_OMAP2.

Note that this will only allow compiling in omap4 with omap2 and
omap3. It will not yet make omap4 boot with multi-omap.

Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>