ARM: OMAP4: use CLK_SET_RATE_PARENT for dss_dss_clk

Set CLK_SET_RATE_PARENT flag for dss_dss_clk so that the DSS's fclk can
be configured without the need to get the parent of the fclk.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>