tegra: spi: add fdt support to tegra SPI SFLASH driver

Add support for configuring tegra SPI driver from devicetree.
Support is keyed off CONFIG_OF_CONTROL.  Add entry in seaboard dts
file for spi controller to describe seaboard spi.

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