dt-bindings: Add SpinalHDL vendor

Adds SpinalHDL as vendor. This project provides FPGA IPs including
VexRiscV CPU.

Signed-off-by: Sebastien Van Cauwenberghe <svancau@gmail.com>
Link: https://lore.kernel.org/r/20211011100531.443157-1-svancau@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>