clk: ti: am33xx: add set-rate-parent support for display clkctrl clock

Display driver assumes it can use clk_set_rate for the display clock
via set-rate-parent mechanism, so add the flag for this to it.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Reported-by: Jyri Sarha <jsarha@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Tested-by: Jyri Sarha <jsarha@ti.com>