tegra: clock: Split the clock source code into a separate function

tegra: clock: Split the clock source code into a separate function

Create a function which sets the source clock for a peripheral, given
the number of mux bits to adjust. This can then be used more generally.
For now, don't export it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>