OMAP2PLUS: DSS2: FEATURES: Function to Provide the max fck supported

The maximum supported frequency for DSS has increased from 173 to 186 Mhz on
OMAP4.

Introduce a dss feature function to get the max_fck to replace DISPC_MAX_FCK
macro.

Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>