overlays: Add 'combine' option to i2c overlays

The i2c0-bcm2708 and i2c1-bcm2708 overlays allow the I2C pin usage to
be changed. The names also suggest they revert to the old i2c_bcm2708
driver, but they don't. The newer i2c_bcm2835 driver forces
transactions to be combined where possible, but this breaks some
devices.

Add an option to disable transaction combining, which is currently
implemented by reverting to the old driver.

See: https://github.com/raspberrypi/firmware/issues/828



Signed-off-by: Phil Elwell <phil@raspberrypi.org>