spi: add spi_tegra driver

spi: add spi_tegra driver

v2 changes:
  from Thierry Reding:
    * add "select TEGRA_SYSTEM_DMA" to Kconfig
  from Grant Likely:
    * add oneline description to header
    * inline references to DRIVER_NAME
    * inline references to BUSY_TIMEOUT
    * open coded bytes_per_word()
    * spi_readl/writel -> spi_tegra_readl/writel
    * move transfer validation to spi_tegra_transfer
    * don't request_mem_region iomem as platform bus does that for us
    * __exit -> __devexit

v3 changes:
  from Russell King:
    * put request_mem_region back int
  from Grant Likely:
    * remove #undef DEBUG
    * add SLINK_ to register bit defines
    * remove unused bytes_per_word
    * make spi_tegra_readl/writel static linine
    * various refactoring for clarity
    * mark err if BSY bit is not cleared after 1000 retries
    * move spinlock to protect setting of RDY bit
    * subsys_initcall -> module_init

v3 changes:
  from Grant Likely:
    * update spi_tegra to use PTR_ERRless dma API

Signed-off-by: Erik Gilling <konkers@android.com>
Cc: Thierry Reding <thierry.reding@avionic-design.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: Russell King <linux@arm.linux.org.uk>