usb: gadget: add infrastructure support for super-speed function suspend

The USB 3.0 specification defines a new 'Function Suspend' feature.
This feature enables the USB host to put inactive composite device
functions in a suspended state even when the device itself is not
suspended. This patch extends the existing framework of USB gadget
to properly support the 'Function Resume' and 'Function Remote Wakeup'
related features.

Change-Id: I51713eac557eabc7b465d161377c09d4b6afa152
Signed-off-by: Danny Segal <dsegal@codeaurora.org>