dt-bindings: mfd: syscon: Add the pbus-csr node for Airoha EN7581 SoC

Introduce pbus-csr document bindings in syscon.yaml for EN7581 SoC.
The airoha pbus-csr block provides a configuration interface for the PBUS
controller used to detect if a given address is accessible on PCIe
controller.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20250312-en7581-pbus_csr-binding-v3-1-7bd9f00a25a4@kernel.org


Signed-off-by: Lee Jones <lee@kernel.org>