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

This change clubs the various register read/write in xfer_in in single
SPI message. Avoids using individual messages for every SPI command 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: I48aefc62bcbf899d8f787dda34fe20841858ab5b
Signed-off-by: Tarun Gupta <tarung@codeaurora.org>