cyclic: Add 'cyclic list' and 'cyclic demo' commands

cyclic: Add 'cyclic list' and 'cyclic demo' commands

This patch adds the cyclic command, which currently only supports the
'list' subcommand, to list all currently registered cyclic functions.
Here an example:

=> cyclic list
function: cyclic_demo, cpu-time: 7010 us, frequency: 99.80 times/s
function: cyclic_demo2, cpu-time: 1 us, frequency: 1.13 times/s

As you can see, the cpu-time is accounted, so that cyclic functions
that take too long might be discovered. Additionally the frequency is
logged.

The 'cyclic demo' commands registers the cyclic_demo() function to
be executed all 'cycletime_ms' milliseconds. The only thing this
function does is delaying by 'delay_us' microseconds.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>