commands: Add clk commands ...

This adds some basic commands to manupulate clocks on the command
line. Right now we have:

- clk_set_parent
- clk_set_rate
- clk_enable
- clk_disable
- clk_dump

The commands work based on the clock name, otherwise they behave like
the Linux Kernel C functions. clk_dump will dump the clocktree to the
console.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>