emul: Add an emulator for the Atmel AT24 series

This emulator supports basic reads and writes with 8-bit addressing.
It connects itself to any AT24 devices it finds in the device tree. The
I2C emulation controller driver is used to direct I2C messages from the
AT24 driver to the AT24 emulator.

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