docs: usb: rename files to .rst and add them to drivers-api

While there are a mix of things here, most of the stuff
were written from Kernel developer's PoV. So, add them to
the driver-api book.

A follow up for this patch would be to move documents from
there that are specific to sysadmins, adding them to the
admin-guide.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/Kconfig

@@ -114,7 +114,7 @@ config USB_EHCI_HCD
 	  Controller Driver or UHCI (for Via motherboards) Host Controller
 	  Driver too.
 
-	  You may want to read <file:Documentation/usb/ehci.txt>.
+	  You may want to read <file:Documentation/usb/ehci.rst>.
 
 	  To compile this driver as a module, choose M here: the
 	  module will be called ehci-hcd.