ARM: imx6: add missing twd_clk for imx6q clock

With commit 5def51b0 (ARM: 7211/1: smp_twd: get the rate from a clock)
hitting mainline, if we do not have a twd_clk for lookup, we will see
the following error message in boot log.

 smp_twd: clock not found: -2

Actually we should add this clock regardless of the error message,
so that we can:

 * Avoid the local timer calibrating at boot time
 * Make the local timer cpufreq aware on imx6q

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