dm: i2c: Add a command to adjust the offset length

I2C chips can support a register offset, with registers accessible by
sending this offset as the first part of any read or write transaction.
Most I2C chips have a single byte offset, thus the offset length is 1.
This provides access for up 256 registers.

However other offset lengths are supported, including 0.

Add a command to provide access to the offset length from the command
line. This allows the offset length to be read or written.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Christophe Ricard<christophe-h.ricard@st.com>
Reviewed-by: Heiko Schocher <hs@denx.de>