spi/tegra: depend instead of select TEGRA_SYSTEM_DMA

It's unlikely that anyone ever wants to turn off SYSTEM_DMA, but just in
case, it makes more sense to have the driver depend on it than select it.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>