[U-Boot] Introduce clk framework

[Peng Fan]

Hi,

Since we are moving towards to use dtb and driver model,
we still lack of a clk framework.

I just have an idea to introduce linux clk framework to u-boot, then
in driver code, we can do as linux "clk_enable/clk_disable/clk_set_freq".

I'd like to hear more voice on clk framework before I began porting the clk framework
from kernel to U-Boot. Or redesign a simple one, if the kernel one is too complicated?

Thanks,
Peng.