ARM: dts: ls1021a: add pix clock to DCU dts node

The DCU IP has distinct clock inputs for register access and the
pixel clocks, at least in some implementations. LS1021a seems to
use the same clock, therefore specify the same clock for "dcu"
and "pix".

Signed-off-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>