ARM: imx6: set initial power mode in pm function

Rather than setting initial low-power mode in every single i.MX6 clock
initialization function, we should really do that in pm code.  Let's
move imx6q_set_lpm(WAIT_CLOCKED) call into imx6_pm_common_init().

While at it, let's rename the function to imx6_set_lpm() since it's
actually common for all i.MX6 SoCs.

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