sandbox: Disable I2C emulators in SPL

These cannot work with of-platdata since they currently need the
devicetree at runtime. Disable the emulators and the sandbox I2C driver
that needs them. We can enable these later, if needed for testing.

Switch the of_plat_parent test over to use a simple bus instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>