clk: tegra: Make tegra_clk_pll_params __ro_after_init

These structures are only passed to the functions tegra_clk_register_pll,
tegra_clk_register_pll{e/u} or tegra_periph_clk_init during the init
phase. These functions modify the structures only during the init phase
and after that the structures are never modified. Therefore, make them
__ro_after_init.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Acked-By: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>