clk: si5341: Add silabs,iovdd-33 property

Add a property to allow specifying that the external I2C IO pins are using
3.3V voltage thresholds rather than 1.8V.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Link: https://lore.kernel.org/r/20210325192643.2190069-9-robert.hancock@calian.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>