clk: tegra: pll: Add dyn_ramp callback

Add a callback to the pll_params for custom dynamic ramping
functions which can be specified per PLL.

Reviewed-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Bill Huang <bilhuang@nvidia.com>
Signed-off-by: Rhyland Klein <rklein@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>