dt-bindings: add vendor prefix for exegin

Add vendor prefix for Exegin Technologies Limited.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210816064416.1630674-6-claudiu.beznea@microchip.com