sandbox: i2c: Support i2c emulation with of-platdata

At present the i2c emulators require access to the devicetree, which is
not possible (by design) with of-platdata.

Add a way for drivers to record the of-platdata index of their emulator,
so that we can still find the emulator.

This allows i2c emulation to work with of-platdata.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>