ARM i.MX53: set CLK_SET_RATE_PARENT flag on the tve_ext_sel clock

Use imx_clk_mux_flags to set the appropriate flags for the TVE
selector clock. This is needed so tve_clk rate changes can propagate
up to pll4_sw.

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