This is needed because the configuration is different and the i2c-designware
cannot be master/slave without a reset. To resolve that I added this property
to bind it as a slave when needed.

Aww, pity that the HW can't do that. Do you have details why?

If that is really a HW limitation, then I'd suggest having a seperate
driver for slave-only mode so we can differentiate by compatible
strings.