mmc: tegra: support SDHCI SPEC 300

Tegra3 SDHOST controller doesn't advertise v3.00 support by default.
This support has to be enabled by configuring a vendor register in
the tegra3 sd host address space.

Signed-off-by: Pavan Kunapuli <pkunapuli@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Chris Ball <cjb@laptop.org>