OMAP: DSS2: DSI: Add DSI pad muxing support

Add dsi_mux_pads function pointer to omap_dss_board_info, and use the
function pointer in DSI code to configure the DSI pads either to normal
DSI operation, or to pull down when in ULPS.

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