clk: tegra: Add binding for the Tegra124 DFLL clocksource

The DFLL is the main clocksource for the fast CPU cluster on Tegra124
and also provides automatic CPU rail voltage scaling as well. The DFLL
is a separate IP block from the usual Tegra124 clock-and-reset
controller, so it gets its own node in the device tree.

Signed-off-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Signed-off-by: Mikko Perttunen <mikko.perttunen@kapsi.fi>
Acked-by: Michael Turquette <mturquette@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>