arm/tegra: Delete tegra_init_clock()

tegra_init_clock() is written to call tegra2_init_clocks(), which only
exists if Tegra20 support is enabled. This breaks the build of a
Tegra30-only kernel.

tegra_init_clock() isn't actually used any more; tegra20_init_early()
calls tegra2_init_clocks() directly. So, just delete this function.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>