ARM: imx6: Derive spdif clock from pll3_pfd3_454m

SPDIF can derive a TX clock for playback from one of its clock sources --
spdif root clock to match its supporting sample rates. So this patch set
the spdif root clock's parent to pll3_pfd3_454m since the pll3_pfd3_454m
can approximately meet its sample rate requirement.

Signed-off-by: Nicolin Chen <Guangyu.Chen@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>