devicetree: bindings: Add vendor prefix for Kosagi

Add a vendor prefix for Sutajio Ko-Usagi PTE Ltd., which goes by the
more common name of Kosagi.

Signed-off-by: Sean Cross <xobs@kosagi.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Rob Herring <robh@kernel.org>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>