dt-bindings: vendor-prefixes: Document the 'pciclass' prefix

dt-bindings: vendor-prefixes: Document the 'pciclass' prefix

The "pciclass" is an existing prefix used to identify the PCI bridge
devices, but it is not a vendor prefix. So document it in the non-vendor
prefix list.

Tested-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20250116-pci-pwrctrl-slot-v3-4-827473c8fbf4@linaro.org
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>