dt-bindings: Add vendor prefix for Opal Kelly Inc

Opal Kelly is a manufacturer of FPGA Integration Modules.
Signed-off-by: Tom McLeod <tom.mcleod@opalkelly.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -247,6 +247,7 @@ olimex	OLIMEX Ltd.
 onion	Onion Corporation
 onnn	ON Semiconductor Corp.
 ontat	On Tat Industrial Company
+opalkelly	Opal Kelly Incorporated
 opencores	OpenCores.org
 option	Option NV
 ORCL	Oracle Corporation