of: Add vendor prefix for Arasan

PATA(pata_arasan_cf.c) and SDHCI(sdhci-of-arasan.c) drivers
are already using this prefix.
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -20,6 +20,7 @@ amlogic	Amlogic, Inc.
 ams	AMS AG
 amstaos	AMS-Taos Inc.
 apm	Applied Micro Circuits Corporation (APM)
+arasan	Arasan Chip Systems
 arm	ARM Ltd.
 armadeus	ARMadeus Systems SARL
 asahi-kasei	Asahi Kasei Corp.