dt-bindings: vendor-prefixes: add Huawei

The vendor prefix for Huawei [1] is used in device trees [2][3], but was
not documented so far. Add it to the schema to document it.

[1] https://www.huawei.com/en/


[2] arch/arm64/boot/dts/qcom/msm8916-huawei-g7.dts
[3] arch/arm64/boot/dts/qcom/msm8994-angler-rev-101.dts

Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220113103005.GA4421@standask-GA-A55M-S2HP