dt-bindings: vendor-prefixes: add Sipeed

Shenzhen Sipeed Technology Co., Ltd. is a company focused on development
kits, which also contains rebranded Lichee Pi series.

Add its vendor prefix binding.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Rob Herring <robh@kernel.org>