clk: add support for setting clk rate from cmdline

Add new clk subcommand "clk setfreq", for setting up a clock rate
directly from u-boot cmdline. This is handy for any debugging purposes
towards clocks.

Acked-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Tero Kristo <kristo@kernel.org>