Add new silabs,xaxb-ext-clk and silabs,iovdd-33 properties.

Changed vdd-supply on top-level node to optional since it is not actually
used by the driver.

Removed vdd-supply from output sub-nodes, as it was not supported by the
driver and it is not easily possible to support this in that location with
the kernel regulator infrastructure. Changed to have vddoX-supply
attributes for each output on the top-level device node.
Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Link: https://lore.kernel.org/r/20210325192643.2190069-2-robert.hancock@calian.comReviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>