Clocks are an important feature of platforms and have become increasing
complex with time. Most modern SoCs have multiple PLLs and dozens of clock
dividers which distribute clocks to on-chip peripherals.

Some SoC implementations have a clock API which is private to that SoC family,
e.g. Tegra and Exynos. This is useful but it would be better to have a
common API that can be understood and used throughout U-Boot.

Add a simple clock API as a starting point. It supports querying and setting
the rate of a clock. Each clock is a device. To reduce memory and processing
overhead the concept of peripheral clocks is provided. These do not need to
be explicit devices - it is possible to write a driver that can adjust the
I2C clock (for example) without an explicit I2C clock device. This can
dramatically reduce the number of devices (and associated overhead) in a
complex SoC.

Clocks are referenced by a number, and it is expected that SoCs will define
that numbering themselves via an enum.

Signed-off-by: Simon Glass <sjg@chromium.org>

Command provides just dump subcommand for showing clock
frequencies in a soc.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Stefano Babic <sbabic@denx.de>