Fix display of hex char literals in device tree source format

[StekiKun]

The format specifier PRIx8 for unsigned integers was used with a
signed character parameter, yielding unexpected results with values > 0x7F.

If you run for instance dtc -I dts -O dts tests/escapes.dts, you can notice
that the value of the escape-str-2 property, which is initially "\xde\xad\xbe\xef", is not
printed correctly:

/dts-v1/;

/ {
	compatible = "test_string_escapes";
	escape-str = "nastystring: \a\b\t\n\v\f\r\\\"";
	escape-str-2 = "\xffffffde\xffffffad\xffffffbe\xffffffef";
};

This happens because the signed character is type-converted to an int, which should not be used with the %x format specifier.
I fixed this by casting to an unsigned char; another possible way is to add hh to the format specifier but I find the former more self-explanatory.

For a bit of context, I found this as I was attempting to "canonize" DTS files by running dtc -I dts -O dts until it reached a fix-point (it so happens that resolution of path and label refs leave an extra space -- due to the markers -- in data blobs, so one must run dtc twice on a file to reach a stable result).

[dgibson]

Fix looks good, but I'll need a Signed-off-by line as described in the new CONTRIBUTING.md file.