How to put comments in device tree source files

Does the default c way of using comments work?

Yes, use /* comment */.

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ADDENDUM

I've also used C preprocessor #if 0 and #endif directives to quickly disable (or enable) multiple lines of a node during testing.
I have not seen any conditional directives in DT files elsewhere, so such use may be frowned upon.

Here is what devicetree documentation says:

The format of the .dts "source" file is "C" like, supports C and C++ style comments.

So you can use both

/* multiline (C style)
   comments */

and

// single line (C++ style) comments,

although the latter is not commonly used for some reason.