usb: gadget: Add support for DMA mapping optimization of request buffers

In order to improve performance, this patch enabled mapping of USB request
buffers in advance, before queueing them to the HW. This can be used when
there is a pool or pre-mapped request buffers so we don't need to map it
again every time the request gets queued.

Change-Id: Ic3a6db749343ef1345e0816210517755aee055b3
Signed-off-by: Danny Segal <dsegal@codeaurora.org>