OMAP2PLUS: DSS2: Add clock sources to dss device clock configuration

Add enum dss_clock_source in dssdev.clocks struct so that the clock sources can
be specified in the board file.
Replace hard coded clock sources in dsi.c, dpi.c and replace them with the new
clock source members in dssdev.clocks. Modify the sdp4430_lcd_device struct in
board-4430sdp.c to specify clock sources for DISPC_FCLK, LCD1_CLK and DSI1_FCLK.

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