ARM: config: Set CONFIG_USB_GADGET_STORAGE_NUM_BUFFERS to 4

Set the number of storage pipeline buffers to 4 for a
better read/write performance on mass_storage.

Change-Id: I510beae3d6914bffdd50957025d68cd7d6299a64
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>