i2c: OMAP: Add DT support for i2c controller

Add initial DT support to retrieve the frequency using a
DT attribute instead of the pdata pointer if of_node exist.

Add documentation for omap i2c controller binding.

Based on original patches from Manju and Grant.

Signed-off-by: Benoit Cousson <b-cousson@ti.com>
Cc: Ben Dooks <ben-linux@fluff.org>
Reviewed-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Kevin Hilman <khilman@ti.com>