dt-bindings: i3c: Describe Silvaco master binding

Silvaco provide a dual-role I3C master.

Description is rather simple: it needs a register mapping, three
clocks and an interrupt.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210121101808.14654-5-miquel.raynal@bootlin.com