As MMIO is placed at fixed physical addressed, and does not need to be
backed by real RAM physical pages, there's no need to use PhysicalPage
instances to track their pages.
This results in slightly reduced allocations, but more importantly
makes MMIO addresses which end up after the normal RAM ranges work,
like 64-bit PCI BARs usually are.
This adds a simple EHCI driver that currently only interrogates the
device and checks if all ports are addressable via associated legacy
controllers (companion controllers), and warns if this is not the case.
This also adds a lot of the other data structures needed for actually
driving the controller, but these are currently not hooked up to
anything.
To test this run with `SERENITY_EXTRA_QEMU_ARGS="--device usb-ehci"`
or the q35 machine type
