OMAP2PLUS: DSS2: Generalize external clock names in struct dss of dss.c

The dss struct in dss.c has omap2/3 specific clock names. Making them generic,
to increase readability and extendability.

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