arm/tegra: prepare clock code for multiple tegra variants

Rework the tegra20 clock code to support multiple tegra variants :

 * remove tegra2_periph_reset_assert/tegra2_periph_reset_deassert. This
   functionality should be in clock.c.

 * remove tegra_sdmmc_tap_delay and export tegra2_sdmmc_tap_delay
   directly. This feature is handled inside the sdmmc block from tegra30
   onwards. So there is no need for support in the clock code beyond
   tegra20. There are no in tree users of this function.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Colin Cross <ccross@android.com>
Signed-off-by: Olof Johansson <olof@lixom.net>