ARM: tegra: clk_m is the architected timer source clock

While clk_m and the oscillator run at the same frequencies on Tegra114
and Tegra124, clk_m is the proper source for the architected timer. On
more recent Tegra generations, Tegra210 and later, both the oscillator
and clk_m can run at different frequencies. clk_m will be divided down
from the oscillator.

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