tegra: add SPI SLINK driver

Add driver for tegra SPI "SLINK" style driver.  This controller is
similar to the tegra20 SPI "SFLASH" controller.  The difference is
that the SLINK controller is a genernal purpose SPI controller and the
SFLASH controller is special purpose and can only talk to FLASH
devices.  In addition there are potentially many instances of an SLINK
controller on tegra and only a single instance of SFLASH.  Tegra20 is
currently ths only version of tegra that instantiates an SFLASH
controller.

This driver supports basic PIO mode of operation and is configurable
(CONFIG_OF_CONTROL) to be driven off devicetree bindings.  Up to 4
devices per controller may be attached, although typically only a
single chip select line is exposed from tegra per controller so in
reality this is usually limited to 1.

To enable this driver, use CONFIG_TEGRA_SLINK

Signed-off-by: Allen Martin <amartin@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>