dt-bindings: vendor-prefixes: add SoChip

Shenzhen SoChip Technology Co., Ltd. is a hardware vendor that produces
EVBs with Allwinner chips. There's also a SoC named S3 that is developed
by Allwinner (based on Allwinner V3/V3s) but branded SoChip.

Add the vendor prefix for SoChip.

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