i2c-ocores: add some device tree documentation

This puts some documentation for the device tree configuration at the head
of the driver file.  Hopefully this can get moved to a common area for this
type of documentation at a later date; unfortunately, there isn't really
such a place in the kernel tree at this time.

Furthermore, the regstep and clock-frequency parameters are really bus
parameters and should probably be passed to the driver in a better way.
Consider that a TODO.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>