ARM: tegra: clocks: separate tegra_clk_32k_ops from Tegra20 and Tegra30

Currently the tegra20 and tegra30 share the same symbol for
tegra_clk_32k_ops. This will cause a compile error when building
a tegra20-only kernel image. Add tegra_clk_32k_ops for tegra20 and
modify tegra30_clk_32k_ops for tegra30.

Signed-off-by: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>