mmc: tegra: use bus-width property instead of support-8bit

Update the driver to parse the new unified bus-width property introduced
in commit 7f217794

 "mmc: dt: Consolidate DT bindings", instead of the legacy
support-8bit property.

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