arm/dts: dra7xx: add 'ti,set-rate-parent' for dss_dss_clk

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 'dss_dss_clk' clock
node, which is only a gate clock, allowing the setting of the clock rate
to propagate to the PLL.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: devicetree@vger.kernel.org
Acked-by: Tero Kristo <t-kristo@ti.com>