spi: tegra: sequence compatible strings as per preference

Sequence compatible list for tegra20-slink driver to first
look for Tegra30 and then Tegra20. Tegra30 have additional
feature in HW which need to be utilize if it is provided from DT.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>