ARM i.MX53: make tve_ext_sel propagate rate change to PLL

This is needed so the Television Encoder driver can set the rate
on tve_clk and have it propagated up to pll4_sw.

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