spi: remove completely broken Tegra driver

The current SPI driver has many issues. Examples are:

* Segfaulting on most transfers due to expecting all transfers to have
  both RX and TX buffers.
* Hanging on TX transfers since the whole driver flow is driven by RX
  DMA completion, but the HW is only told to enable RX for RX transfers.
* Use of clk_disable_unprepare() from atomic context.
* Once those and other minor issues are fixed, the driver still doesn't
  actually work.
* The driver also implements a deprecated API to the SPI core.

For this reason, simply remove the driver completely. This has two
advantages:

1) This will remove the last use of Tegra's <mach/dma.h>, which will
   allow that file to be removed, which is required for single zImage
   work.

2) The downstream driver is significaly different from the current
   code. I believe a patch to re-add the downstream driver (with
   appropriate cleanup) will be much simpler to review if it's a new
   file rather than randomly interspered with essentially unrelated
   existing code.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>