emul: i2c: Add support for I2C emulators

Add an emulation controller which routes I2C traffic to attached
emulators depending on the I2C address selected. This allows drivers
for I2C peripherals to be tested on systems that don't have that
peripheral attached, with the emulator handling the I2C traffic.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>