PCI: designware: Explain why we don't program ATU for some platforms

Some platforms don't support ATU, e.g., pci-keystone.c.  These platforms
use their own address translation component rather than ATU, and they
provide the rd_other_conf and wr_other_conf methods to program the
translation component and perform the access.

Add a comment to explain why we don't program the ATU for these platforms.

[bhelgaas: changelog]
Signed-off-by: NJisheng Zhang <jszhang@marvell.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>

drivers/pci/host/pcie-designware.c

@@ -517,6 +517,11 @@ int dw_pcie_host_init(struct pcie_port *pp)
 	if (pp->ops->host_init)
 		pp->ops->host_init(pp);
 
+	/*
+	 * If the platform provides ->rd_other_conf, it means the platform
+	 * uses its own address translation component rather than ATU, so
+	 * we should not program the ATU here.
+	 */
 	if (!pp->ops->rd_other_conf)
 		dw_pcie_prog_outbound_atu(pp, PCIE_ATU_REGION_INDEX1,
 					  PCIE_ATU_TYPE_MEM, pp->mem_base,