clk: tegra: Re-factor T210 PLLX registration

Tegra210 PLLX uses the same sequences than then PLLC instances. So there
is no need to have a special registration function and ops struct for it.
Simplify the code by changing all references to the Tegra210 PLLX
registration function to the Tegra210 PLLC registration function and
avoid duplicate functionality.

Based on work by Alex Frid <afrid@nvidia.com>

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>