ARM: tegra: Implement clk_m

On currently supported SoCs, clk_m always runs at the same frequency as
the oscillator input. However newer SoC generations such as Tegra210 no
longer have that restriction. Prepare for that by separating clk_m from
the oscillator clock and allow SoC code to override the clk_m rate.

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