dm: i2c: Add a function to find out the chip offset length

We can currently set this but there is no API function to get it. Add one.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Heiko <Schocher&lt;hs@denx.de>