doc: add bind/unbind command documentation

Add documentation in doc/drivel-model for the bind/unbind command.
Part of this documentation is extracted from original patch commit
message:
commit 49c752c9 ("cmd: Add bind/unbind commands to bind a device to a driver from the command line")

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>