clk: si5341: Add silabs,xaxb-ext-clk property

Add a property to allow specifying that the device XA/XB pins are used for
an external clock input rather than for a clock crystal.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>
Link: https://lore.kernel.org/r/20210325192643.2190069-8-robert.hancock@calian.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>