MXC : update i.MX21 clock support for USB host.

* Use correct clkdev style usb clock name
* Implement rate setting for USB clock
* Introduce _clk_generic_round_rate to factorize the (now 3) uses of rounding code.

Signed-off-by: Martin Fuzzey <mfuzzey@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>