usb gadget serial: split out generic serial function

Split out the generic serial support into a "function driver".  This
closely mimics the ACM support, but with a MUCH simpler control model.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/Documentation/DocBook/gadget.tmpl b/Documentation/DocBook/gadget.tmpl
index b7e266c..ea3bc95 100644
--- a/Documentation/DocBook/gadget.tmpl
+++ b/Documentation/DocBook/gadget.tmpl
@@ -557,6 +557,7 @@
 </para>
 
 !Edrivers/usb/gadget/f_acm.c
+!Edrivers/usb/gadget/f_serial.c
 
 </sect1>