Currently, Tegra peripheral drivers control two aspects of their HW module
clock(s):

1) The clock enable/rate for the peripheral clock itself.

2) The system-level clock tree setup, i.e. the clock parent.

Aspect 1 is reasonable, but aspect 2 is a system-level decision, not
something that an individual peripheral driver should in general know
about or influence. Such system-level knowledge ties the driver to a
specific SoC implementation, even when they use generic APIs for clock
manipulation, since they must have SoC-specific knowledge such as parent
clock IDs. Limited exceptions exist, such as where peripheral HW is
expected to dynamically switch between clock sources at run-time, such
as CPU clock scaling or display clock conflict management in a multi-head
scenario.

This patch enhances the Tegra core code to perform system-level clock
tree setup, in a similar fashion to the Linux kernel Tegra clock driver.
This will allow future patches to simplify peripheral drivers by removing
the clock parent setup logic.

This change is required prior to converting peripheral drivers to use the
standard clock APIs, since:

1) The clock uclass doesn't currently support a set_parent() operation.
Adding one is possible, but not necessary at the moment.

2) The clock APIs retrieve all clock IDs from device tree, and the DT
bindings for almost all peripherals only includes information about the
relevant peripheral clocks, and not any potential parent clocks.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>