of: Add vendor prefix for Sitronix

The stock ticker for Sitronix is just a number.
"sitronix,st1232" is already in use for the Sitronix st1232 touchscreen
controller on Atmark Techno Armadillo 800 EVA.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -127,6 +127,7 @@ silabs	Silicon Laboratories
 simtek
 sii	Seiko Instruments, Inc.
 sirf	SiRF Technology, Inc.
+sitronix	Sitronix Technology Corporation
 smsc	Standard Microsystems Corporation
 snps 	Synopsys, Inc.
 solidrun	SolidRun