mmc: tegra: convert to device tree support only

When compiling a kernel that supports only Tegra30 and not Tegra20,
the SDHCI driver will fail to compile since the of_match_device() failure
fallback code unconditinally references soc_data_tegra20, which is only
compiled in when Tegra20 support is enabled.

In Kernel 3.7, both Tegra20 and Tegra30 will only support booting using
device tree, Hence, there is never a need to fall back to using
soc_data_tegra20 if of_mach_device() fails. Instead, make this case an
error. This removes the reference to soc_data_tegra20, and hence solves
the compile failure.

Signed-off-by: Joseph Lo <josephl@nvidia.com>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Chris Ball <cjb@laptop.org>