OMAP2PLUS: DSS2: Change enum "dss_clk_source" to "omap_dss_clk_source"

Change enum dss_clk_source to omap_dss_clock_source and move it to
'plat/display.h'. Change the enum members to attach "OMAP_" in the beginning.
These changes are done in order to specify the clock sources for DSS in the
board file.

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