max3107 & device tree

Hi,

I have a max3107 on my board, I'm trying to configure the device tree,
but it looks like that I need a clock :

http://lxr.free-electrons.com/source/Documentation/devicetree/bindings/serial/maxim,max310x.txt?v=4.1

clocks: phandle to the IC source clock. clock-names: Should be "xtal"
if clock is an external crystal or "osc" if an external clock source
is used.

And I don't know what to put for the clock.

I would like to use xtal because I've a an external clock (quartz)
that is driving the max3107 but I don't know what to put for the
clocks = ?? I don't have clk20m referenced in the file
am33xx-clocks.dtsi .....

the frequency of my quartz is 1.8432MHz

any idea what I should do ?