ARM: OMAP2+: board-generic: Add DT support to generic board

Re-cycle the original board-generic.c file to support Device Tree
for every OMAP2+ variants.

The current approach is an intermediate step before having only
one machine descriptor that will use some generic DT aware
functions.

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