xhci: Allow the XHCI USB controller to be enabled for coreboot.

Initial tests look promising and at least two different real-world
chipsets show limited functionality with the support enabled.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
xhci: Don't use a dummy endpoint count in configure command.

At least some real-world controllers expect the endpoint count in the
inctx to be accurate, so set it to the pipe currently being activated.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
xhci: Report the "protocol defined" and PSIC bits of extended capabilities.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>