OMAP2PLUS: DSS2: Use dss features to get clock source names of current OMAP

Clock source names vary across OMAP2/3 and OMAP4, the clock source enum
names have been made generic in the driver, but for purposes of debugging
and dumping clock sources, it is better to preserve the actual TRM name of
the clock.

Introduce a dss feature function 'dss_feat_get_clk_source_name()' which
returns a string with the TRM clock name for the current OMAP in use. The OMAP
specific name is printed along the generic name within brackets.

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