ARM: dts: am43xx-clocks.dtsi: add ti, set-rate-parent to display clock path

We need set-rate-parent flags for the display's clock path so that the
DSS driver can change the clock rate of the PLL.

This patchs adds the ti,set-rate-parent flag to disp_clk and
dpll_disp_m2_ck clock nodes.

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