ARM: OMAP2+: kconfig: Enable devicetree by default for OMAP2+ systems

devicetree will become the mandatory boot method for OMAP2+.
In order to avoid cluttering the OMAP code with #ifdef CONFIG_OF,
select USE_OF by default for every OMAP2+ systems.
Select as well the APPENDED_DTB and ATAG_DTB_COMPAT to allow legacy
boot loader to keep working properly.

Enable PROC_DEVICETREE as well.

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