OMAP4: DSS2: clocks: Add ick as dummy clock

DSS code uses ick as one of the clocks in clk_get/clk_put. OMAP4 clock database
doesn't have ick for DSS, so adding ick as dummy clock.

This is needed for backward compatibility with OMAP2/3.

Once pm_runtime* APIs get introduced in DSS, this will be revisited.

Signed-off-by: Sumit Semwal <sumit.semwal@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>