i2c: omap: Fix missing device tree flags for omap2

As we claim to support device tree for mach-omap2, we
should have the necessary flags in the driver to make it
usable.

Cc: linux-i2c@vger.kernel.org
Acked-by: Wolfram Sang <wsa@the-dreams.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>