OMAP2PLUS:DSS2: add opt_clock_available in pdata

Provide a function in pdata to allow dss submodules to check if a given
clock is available on a platform as an optional clock.

Signed-off-by: Senthilvadivu Guruswamy <svadivu@ti.com>
(based on implementation from Senthil)

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