OMAP2PLUS: DSS2: Make members of dss_clk_source generic

The enum members of 'dss_clk_source' have clock source names specific to
OMAP2/3. Change the names to more generic terms such that they now describe
where the clocks come from and what they are used for.

Also, change the enum member names to have "DSS_CLK_SRC" instead of "DSS_SRC"
for more clarity.

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