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>