dt-bindings: PCI: snps,dw-pcie-ep: Add vendor specific reg-name

dt-bindings: PCI: snps,dw-pcie-ep: Add vendor specific reg-name

Considering that some drivers (e.g. pcie-dw-rockchip.c) already use the
reg-name "apb" for the device tree binding in Root Complex mode
(snps,dw-pcie.yaml), it doesn't make sense that those drivers should use a
different reg-name when running in Endpoint mode (snps,dw-pcie-ep.yaml).

Therefore, since "apb" is already defined in snps,dw-pcie.yaml, add it
also for snps,dw-pcie-ep.yaml.

Link: https://lore.kernel.org/linux-pci/20240607-rockchip-pcie-ep-v1-v5-1-0a042d6b0049@kernel.org
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>