ARM: imx: Correct the comments in time.c

The comments were corrected as the following to reflect
the real situation of Freescale MXC timer IP block.
There are totally 4 version of the timer on Freescale i.MX SoCs.

Signed-off-by: Shenwei Wang <shenwei.wang@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>