Move clk code from 'mach-imx' to 'drivers' to keep the code tree
structure closer to that of analogous one from Linux kernel and,
arguably although subjective, to keep 'mach-imx' less cluttered.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>