dt-bindings: vendor-prefixes: add Wingtech

The vendor prefix for Wingtech [1] is used in device tree [2], but was
not documented so far. Add it to the schema to document it.

[1] http://www.wingtech.com/en


[2] arch/arm64/boot/dts/qcom/msm8916-wingtech-wt88047.dts

Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220113103110.GA4488@standask-GA-A55M-S2HP