arm/dt: Tegra: Add support-8bit to SDHCI nodes

For Seaboard's internal eMMC, this makes the difference between a
5.5MB/s and 10.2MB/s transfer rate. On Harmony, there wasn't any
measurable difference on my cheap/slow ~2MB/s card.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>