ARM: dt: Add references to tegra_car clocks

Add references to tegra_car clocks for the basic device nodes. Also remove
the clock-frequency property of the serial node as the UART driver can now
use the clock framework to obtain the frequency.

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>