ARM i.MX5: Add S/PDIF clocks

This patch adds the S/PDIF clocks for i.MX51 and i.MX53. Tested on i.MX53.
The i.MX51 has a second set of spdif_root clock dividers, and on i.MX53
there is an additional input to the spdif_xtal mux.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>