dt-bindings: vendor-prefixes: add Thundercomm

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

[1] https://www.thundercomm.com/


[2] arch/arm64/boot/dts/qcom/sdm845-db845c.dts

Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220113103036.GA4456@standask-GA-A55M-S2HP