clk: ti: clkctrl: add support for CLK_SET_RATE_PARENT flag

Certain clkctrl clocks, notably the display ones, use the
CLK_SET_RATE_PARENT feature extensively. Add support for this flag
to the clkctrl clocks.

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>