pci_dw: fix outbound I/O window configuration

Use viewport "2" instead of "0" and change window type from MEM to IO.
Without these changes the MEM ATU window can be overwritten with the IO one.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Obtained from: Semihalf
Sponsored by: Marvell
Differential revision: https://reviews.freebsd.org/D29516