ARM: dts: omap3-igep: Update to use the TI AM/DM37x processor

Most of the boards are using the TI AM/DM37x processor, there is only a small
quantity of IGEP Processor Boards based on TI OMAP3530. So it's better use the
omap36xx.dtsi include instead of omap34xx.dtsi include. We can add support
for the 34xx based variant later on as needed.

To avoid confusion we have added to the model the (TI AM/DM37x) comment.

Signed-off-by: Enric Balletbo i Serra <eballetbo@gmail.com>
[tony@atomide.com: updated comments for the 34xx to 36xx include change]
Signed-off-by: Tony Lindgren <tony@atomide.com>