ARM: imx6q: remove unneeded clk lookups

Since commit (a94f8ec ARM: imx6q: remove board specific CLKO setup),
a number of clk lookups in imx6q clock driver is no longer needed.
Let's remove them.

The cpu0 lookup is also removed since we are now running imx6 cpufreq
driver and looking up clocks from device tree.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>