Add a sandbox I2C emulation device which emulates a real-time clock. The
clock works off an offset from the current system time, and supports setting
and getting the clock, as well as access to byte-width regisers in the RTC.
It does not support changing the system time.

This device can be used for testing the 'date' command on sandbox, as well
as the RTC uclass.

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