Intel xhci: Support EHCI/xHCI port switching.

The Intel Panther Point chipsets contain an EHCI and xHCI host controller
that shares some number of skew-dependent ports.  These ports can be
switched from the EHCI to the xHCI host (and vice versa) by a hardware MUX
that is controlled by registers in the xHCI PCI configuration space.  The
USB 3.0 SuperSpeed terminations on the xHCI ports can be controlled
separately from the USB 2.0 data wires.

This switchover mechanism is there to support users who do a custom
install of certain non-Linux operating systems that don't have official
USB 3.0 support.  By default, the ports are under EHCI, SuperSpeed
terminations are off, and USB 3.0 devices will show up under the EHCI
controller at reduced speeds.  (This was more palatable for the marketing
folks than having completely dead USB 3.0 ports if no xHCI drivers are
available.)  Users should be able to turn on xHCI by default through a
BIOS option, but users are happiest when they don't have to change random
BIOS settings.

This patch introduces a driver method to switchover the ports from EHCI to
xHCI before the EHCI driver finishes PCI enumeration.  We want to switch
the ports over before the USB core has the chance to enumerate devices
under EHCI, or boot from USB mass storage will fail if the boot device
connects under EHCI first, and then gets disconnected when the port
switches over to xHCI.

Add code to the xHCI PCI quirk to switch the ports from EHCI to xHCI.  The
PCI quirks code will run before any other PCI probe function is called, so
this avoids the issue with boot devices.

Another issue is with BIOS behavior during system resume from hibernate.
If the BIOS doesn't support xHCI, it may switch the devices under EHCI to
allow use of the USB keyboard, mice, and mass storage devices.  It's
supposed to remember the value of the port routing registers and switch
them back when the OS attempts to take control of the xHCI host controller,
but we all know not to trust BIOS writers.

Make both the xHCI driver and the EHCI driver attempt to switchover the
ports in their PCI resume functions.  We can't guarantee which PCI device
will be resumed first, so this avoids any race conditions.  Writing a '1'
to an already set port switchover bit or a '0' to a cleared port switchover
bit should have no effect.

The xHCI PCI configuration registers will be documented in the EDS-level
chipset spec, which is not public yet.  I have permission from legal and
the Intel chipset group to release this patch early to allow good Linux
support at product launch.  I've tried to document the registers as much
as possible, so please let me know if anything is unclear.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>