t210: SPI driver cleanup

1. Get rid of spi_delay - Instead have a tight loop to check for the
spi status
2. The first check for SPI operation complete i.e. FIFOs have been
processed is the SPI_STATUS_RDY bit. Thus, tegra_spi_wait should check
for this bit before reading BLOCK_COUNT or any other fifo count field.
3. Flush both TX and RX FIFOs for SEND and RECV operations for PIO and
DMA.
4. No need to check for rx_fifo_count == spi_byte_count to determine
pio_finish operation. RDY bit should be sufficient to ensure that the
SPI operation is complete. Added assert to ensure we never hit the
case of RDY bit being set, yet rx_fifo_count != spi_byte_count for
PIO.

BUG=chrome-os-partner:41877
BRANCH=None
TEST=Compiles successfully and reboot test runs successfully for 10K+ iterations.

Change-Id: I1adb9672c1503b562309a8bc6c22fe7d2271768e
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: de1515605e17e0c6b81874f9f3c49fd0c1b92756
Original-Change-Id: I5853d0df1bfd6020a17e478040bc4c1834563fe4
Original-Signed-off-by: Furquan Shaikh <furquan@google.com>
Original-Reviewed-on: https://chromium-review.googlesource.com/285141
Original-Reviewed-by: Jimmy Zhang <jimmzhang@nvidia.com>
Original-Tested-by: Jimmy Zhang <jimmzhang@nvidia.com>
Original-Reviewed-by: Furquan Shaikh <furquan@chromium.org>
Original-Tested-by: Furquan Shaikh <furquan@chromium.org>
Original-Commit-Queue: Furquan Shaikh <furquan@chromium.org>
Original-Trybot-Ready: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: http://review.coreboot.org/10947
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

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