OMAP4: DSS2: Using dss_features to set independent core clock divider

Using dss_features to select independent core clock divider and setting
it. Added the register used, to DISPC context save and restore group

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In OMAP4, the minimum DISPC_CORE_CLK required can be expressed as:

	DISPC_CORE_CLK >= max(PCLK1*HSCALE1, PCLK2*HSCALE2, ...)

Where PCLKi is the pixel clock generated by MANAGERi and HSCALEi is the
maximum horizontal downscaling done through MANAGERi

Based on the usecase, core clk can be increased or decreased at runtime
to save power. Such mechanism are not yet implemented. Hence, we set the
core clock divisor to 1, to support maximum range of resolutions
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Signed-off-by: Raghuveer Murthy <raghuveer.murthy@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>