ARM: dts: omap4.dtsi: remove dss_fck

"dss_fck" is a hacky clock, used to work around problems with MODULEMODE
bit handling in DSS hwmods.

These problems have now been solved, so we can remove the dss_fck clock.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit.taneja@gmail.com>
Signed-off-by: Paul Walmsley <paul@pwsan.com>