arm64: dts: ls1028a: declare cache-coherent page table walk feature for IOMMU

The SMMUv2 driver for MMU-500 reads the ARM_SMMU_GR0_ID0 register at
probe time and tries to determine based on the CTTW (Coherent
Translation Table Walk) bit whether this feature is supported.

Unfortunately, it looks like the SMMU integration in the NXP LS1028A has
wrongly tied the cfg_cttw signal to 0, even though the SoC documentation
specifies that "The SMMU supports cache coherency for page table walks
and DVM transactions for page table cache maintenance operations."

Device tree provides the option of overriding the ID register via the
dma-coherent property since commit bae2c2d4 ("iommu/arm-smmu: Sort
out coherency"), and that's what we do here.

Telling struct io_pgtable_cfg that the SMMU page table walks are
coherent with the CPU caches brings performance benefits, because it
avoids certain operations such as __arm_lpae_sync_pte() for PTE updates.

Link: https://lore.kernel.org/linux-iommu/3f3112e4-65ff-105d-8cd7-60495ec9054a@arm.com/Suggested-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm64/boot/dts/freescale/fsl-ls1028a.dtsi

@@ -713,6 +713,7 @@ smmu: iommu@5000000 {
 			reg = <0 0x5000000 0 0x800000>;
 			#global-interrupts = <8>;
 			#iommu-cells = <1>;
+			dma-coherent;
 			stream-match-mask = <0x7c00>;
 			/* global secure fault */
 			interrupts = <GIC_SPI 13 IRQ_TYPE_LEVEL_HIGH>,