dt-bindings: mtd: cdns,hp-nfc: Add iommu property

dt-bindings: mtd: cdns,hp-nfc: Add iommu property

Agilex5 integrates an ARM SMMU (System Memory Management Unit) with
Translation Buffer Units (TBUs) assigned to various peripherals,
including the NAND controller.

The Cadence HP NAND controller ("cdns,hp-nfc") on Agilex5 is behind a
TBU connected to the system's SMMUv3. To support this, the controller
requires an `iommus` property in the device tree to properly configure
address translation through the IOMMU framework.

Adding the `iommus` property to the binding schema allows the OS
to associate the NAND controller with its corresponding SMMU stream ID.
This enables:
- DMA address translation between the controller and system memory
- Memory protection for NAND operations
- Proper functioning of the IOMMU framework in secure or virtualized
  environments

This change documents the IOMMU integration for the NAND controller
on platforms like Agilex5 where such hardware is present.

Signed-off-by: Adrian Ng Ho Yin <adrianhoyin.ng@altera.com>
Signed-off-by: Khairul Anuar Romli <khairul.anuar.romli@altera.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>