ARM: OMAP2+: board-generic: Remove un-needed .atag_offset for DT_MACHINE

Some .atag_offset entries were wrongly added during a merge conflict
resolution in 3.3.
Remove them all, since DT boot does not use that attribute anymore.

Replace as well the #if... by #ifdef for consistency.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Tony Lindgren <tony@atomide.com>