tegra20: rename CONFIG_MACH_TEGRA_GENERIC

Rename CONFIG_MACH_TEGRA_GENERIC to the less confusing CONFIG_TEGRA.
The meaning of the config options is now:

CONFIG_TEGRA - Any tegra chip
CONFIG_TEGRA20 - A tegra20 family chip
CONFIG_TEGRA30 - A tegra30 family chip (not added yet)

Signed-off-by: Allen Martin <amartin@nvidia.com>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Tested-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Tom Warren <twarren@nvidia.com>