dt-bindings: vendor-prefixes: add F(x)tec

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

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


[2] arch/arm64/boot/dts/qcom/msm8998-fxtec-pro1.dts

Signed-off-by: Stanislav Jakubek <stano.jakubek@gmail.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220113102926.GA4388@standask-GA-A55M-S2HP