USB: ice40-hcd: Add optimization to improve write throughput

This change clubs the various register read/write in xfer_out in single
SPI message instead of using multiple messages with this we can avoid
unnecessary context switches. This also implements double buffering to make
use of two FIFO's avaialble for efficient bus bandwidth utilizaion of
USB BUS.

CRs-Fixed: 646320
Change-Id: I583d73b9b61e110b370c97e1b72392c2af0d1cee
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>