ARM: dts: omap5-clocks.dtsi: add ti, set-rate-parent to dss_dss_clk

Add ti,set-rate-parent to dss_dss_clk so that the DSS driver can
set the rate.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>