(WIP, please help me improve this, or just do it better :D)
AcpiOsMapMemory is used for device memory when e.g. an _INI method wants to access physical memory.
However, aarch64 pmap_mapbios is hardcoded to writeback, which is no good.
We need to default to device memory and use EFI memory map attributes to detect different modes.
This fixes XHCI initialization on the Raspberry Pi 4 UEFI firmware (https://github.com/pftf/RPi4/issues/50).