OMAPDSS: Add enum dss_pll_id

In some cases we need global identifiers for the DSS PLLs, for example
when configuring clock muxing on DRA7. For this purpose let's add a
'enum dss_pll_id'.

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

diff --cc drivers/video/fbdev/omap2/dss/dsi.c
Simple merge

diff --cc drivers/video/fbdev/omap2/dss/dss.h
Simple merge

diff --cc drivers/video/fbdev/omap2/dss/hdmi_pll.c
Simple merge