While booting from device tree, imx6q used to provide twd clock lookup
by calling clk_register_clkdev() in clock driver.  However, the commit
bd603455 (ARM: use device tree to get smp_twd clock) forces DT boot to
look up the clock from device tree.  It causes the failure below when
twd driver tries to get the clock, and hence kernel has to calibrate the
local timer frequency.

 smp_twd: clock not found -2
 ...
 Calibrating local timer... 396.13MHz.

Fix the regression by providing twd clock lookup from device tree, and
remove the unused twd clk_register_clkdev() call from clock driver.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>