OpenFirmware: Include <linux/of_i2c.h> from of_i2c.c.

drivers/of/of_i2c.c should include <linux/of_i2c.h> for the prototype
for of_register_i2c_devices().

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Acked-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Jochen Friedrich <jochen@scram.de>
Acked-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>