spi: add Kconfig variable for dual-output read enable

Add a Kconfig variable so that driver code knows whether
or not to use dual-output reads.

Signed-off-by: David Hendricks <dhendrix@chromium.org>

Old-Change-Id: I31d23bfedd91521d719378ec573e33b381ebd2c5
Reviewed-on: https://chromium-review.googlesource.com/177834
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Commit-Queue: David Hendricks <dhendrix@chromium.org>
Tested-by: David Hendricks <dhendrix@chromium.org>
(cherry picked from commit de6869a3350041c6823427787971efc9fcf469b8)

tegra124: implement x2 mode for SPI transfers on CBFS media

This implements x2 mode when reading CBFS media over SPI.

In theory this effectively doubles our throughput, though the initial
results were almost negligibly better. Using a logic analyzer we see
a pattern of 12 clocks, ~70ns delay, 4 clocks, ~310ns delay. So if we
want to see further gains here then we'll probably need to tune AHB
arbitration and utilization to eliminate bubbles/stalls when copying
from APB DMA.

Signed-off-by: David Hendricks <dhendrix@chromium.org>

Old-Change-Id: I33d6ae30923fc42b4dc7103d029085985472cf3e
Reviewed-on: https://chromium-review.googlesource.com/177835
Reviewed-by: Tom Warren <twarren@nvidia.com>
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Commit-Queue: David Hendricks <dhendrix@chromium.org>
Tested-by: David Hendricks <dhendrix@chromium.org>
(cherry picked from commit 29289223362b12e84da5cbb130f285c6b9d314cc)

nyan: turn on dual-output reads for SPI flash

Nyan's SPI chip is capable of dual-output reads, so let's use it.

Signed-off-by: David Hendricks <dhendrix@chromium.org>

Old-Change-Id: I51a97c05aa25442d8ddcc4e3e35a2507d91a64df
Reviewed-on: https://chromium-review.googlesource.com/177836
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Commit-Queue: David Hendricks <dhendrix@chromium.org>
Tested-by: David Hendricks <dhendrix@chromium.org>
(cherry picked from commit 62de0889a9cfc5686800645d05e21e272e4beb5c)

Squashed three commits to enable dual output spi reads for nyan.
Also fixed the spi_xfer interface that has been updated to use bytes
instead of bits.

Change-Id: I750a177576175b297f61e1b10eac6db15e75aa6e
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/6909
Tested-by: build bot (Jenkins)
Reviewed-by: David Hendricks <dhendrix@chromium.org>