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: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>