OMAP2PLUS: DSS2: Generalize naming of PRCM related clock enums in DSS driver

enum dss_clock structure is replaced with generic names that
could be used across OMAP2420, 2430, 3xxx, 44xx platforms.

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>