Non-normative UEFI text: extend 8.2.3 PCIe

[andreiw]

Mention EFI_PCI_IO_PROTOCOL GetBarAttributes to get the host side address for direct BAR access. Explain why - i.e. mapping of 32-bit non-pf MMIO to 64-bit MMIO space on some systems

Existing text around using EFI_PCI_ROOT_BRIDGE_IO_PROTOCOL_CONFIGURATION isn't factually incorrect, just not particularly useful.