OMAPDSS: DSI: add new clock calculation code

Add new way to iterate over DSI PLL and HSDIV clock divisors.
dsi_pll_calc() and dss_hsdiv_calc() provide a generic way to go over
all the divisors, within given clock range. The functions will call a
callback function for each divider set, making the function reusable for
all use cases.

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