usb: xhci: make USB_XHCI_PLATFORM selectable

Right now xhci-plat-hcd can be built when using one of platform specific
drivers only (mvebu/rcar). There shouldn't be such limitation as some
platforms may not require any quirks and may want to just use a generic
driver ("generic-xhci" / "xhci-hcd").

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/Kconfig

@@ -32,7 +32,14 @@ config USB_XHCI_PCI
        default y
 
 config USB_XHCI_PLATFORM
-	tristate
+	tristate "Generic xHCI driver for a platform device"
+	---help---
+	  Adds an xHCI host driver for a generic platform device, which
+	  provides a memory space and an irq.
+	  It is also a prerequisite for platform specific drivers that
+	  implement some extra quirks.
+
+	  If unsure, say N.
 
 config USB_XHCI_MVEBU
 	tristate "xHCI support for Marvell Armada 375/38x"