clk: tegra: Rename Tegra124 EMC clock source file

This code is only used on Tegra124, so rename it accordingly to make it
more consistent with other file names.

While at it, also get rid of the TEGRA_CLK_EMC Kconfig symbol that's
really just an alias for TEGRA124_EMC.

Signed-off-by: Thierry Reding <treding@nvidia.com>