dm: i2c: EEPROM simulator allow tests visibility of addr and offset

Improve i2c EEPROM simulator testing by providing access functions to
check the previous chip addr and offset.

Given that we can now directly test the offsets, also simplified the
offset mapping and allow for wrapping acceses.

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
Reviewed-by: Heiko Schocher <hs@denx.de>