tegra20: spi: move fdt probe to spi_init

Make the tegra20 SPI driver similar to the tegra30 (and soon to be
tegra114) SPI drivers in preparation of common fdt SPI driver front
end.

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