ARM i.MX28: use correct register for setting the rate

_CLK_SET_RATE does not only handle the cpu clock but also other
clocks, so do not hardcode the HW_CLKCTRL_CPU register.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@freescale.com>