ARM: tegra: colibri_t20: rename ac97 label to tegra_ac97

Rename ac97 label to tegra_ac97 to be more in-line with the device tree
binding documentation.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>