PCI: designware-ep: Use aligned ATU window for raising MSI interrupts

Certain platforms like K2G reguires the outbound ATU window to be
aligned. The alignment size is already present in mem->page_size.
Use the alignment size present in mem->page_size to configure an
aligned ATU window. In order to raise an interrupt, CPU has to write
to address offset from the start of the window unlike before where
writes were always to the beginning of the ATU window.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>