dt-bindings: Add vendor prefix for Silvaco

Silvaco, Inc. is an EDA provider of software tools used for process
and device development and for analog/mixed-signal, power IC and
memory design [1].

[1] https://www.silvaco.com/company/profile/profile.html



Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210121101808.14654-4-miquel.raynal@bootlin.com