dm: Add a simple EEPROM driver

There seem to be a few EEPROM drivers around - perhaps we should have a
single standard one? This simple driver is used for sandbox testing, but
could be pressed into more active service.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>