Introduce DSI2 PLL clock sources needed by LCD2 channel and DSI2 Protocol
engine and DISPC Functional clock. Do the following:

- Modify dss_get_dsi_clk_source() and dss_select_dsi_clk_source() to take the
  dsi module number as an argument.
- Create debugfs files for dsi2, split the corresponding debugfs functions.
- Allow DPI to use these new clock sources.
Signed-off-by: Archit Taneja <archit@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>