dt-bindings: vendor-prefixes: Add smi

Add Silicon Mortion Technology Corporation
https://www.siliconmotion.com/



Signed-off-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/c8aaf67e3fcdb7e60632c53a784691aabfc7733e.1704788539.git.ysato@users.sourceforge.jp


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