ARM: tegra: Rename sdhci nodes to mmc

The new json-schema based validation tools require SD/MMC controller
nodes to be named mmc. Rename all references to them.

Cc: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Cc: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>