of: Add Silicon Image vendor prefix

"sil" is the most commonly used abbreviation for Silicon
Image products.
Signed-off-by: NPawel Moll <pawel.moll@arm.com>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -33,6 +33,7 @@ qcom	Qualcomm, Inc.
 ramtron	Ramtron International
 samsung	Samsung Semiconductor
 schindler	Schindler
+sil	Silicon Image
 simtek
 sirf	SiRF Technology, Inc.
 stericsson	ST-Ericsson