ARM: OMAP2+: clock: fix clkoutx2 with CLK_SET_RATE_PARENT

If CLK_SET_RATE_PARENT is set for a clkoutx2 clock, calling
clk_set_rate() on the clock "skips" the x2 multiplier as there are no
set_rate and round_rate functions defined for the clkoutx2.

This results in getting double the requested clock rates, breaking the
display on omap3430 based devices. This got broken when
d0f58bd3

 and related patches were merged
for v3.14, as omapdss driver now relies more on the clk-framework and
CLK_SET_RATE_PARENT.

This patch implements set_rate and round_rate for clkoutx2.

Tested on OMAP3430, OMAP3630, OMAP4460.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>