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>
diff --git a/src/Kconfig b/src/Kconfig
index 071a16e..cce3ad8 100644
--- a/src/Kconfig
+++ b/src/Kconfig
@@ -250,7 +250,7 @@
             Support USB EHCI controllers.
     config USB_XHCI
-        depends on USB && QEMU_HARDWARE
+        depends on USB
         bool "USB XHCI controllers"
         default y