dt-bindings: i2c: mux-gpio: Add 'settle-time-us' property

I2C MUXes described by the i2c-gpio-mux sometimes need a significant
amount of time to switch from a bus to another. When a new bus is
selected, the first I2C transfer can fail if it occurs too early. There
is no way to describe this transition delay that has to be waited before
starting the first I2C transfer.

Add a 'settle-time-us' property that indicates the delay to be
respected before doing the first i2c transfer.

Signed-off-by: Bastien Curutchet <bastien.curutchet@bootlin.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Acked-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>