ARM: OMAP2+: DT 'compatible' tweak for displays

As there is no common panel framework in the kernel, we have OMAP
specific panel drivers. However, the DT data should be generic. This
brings the issue that some other platform could use the same panels, and
would need to create a driver with the same 'compatible' string as the
OMAP driver.

In the long run, we have to get a common panel framework. For the time
being, this patch solves the issue:

At early boot time, we go through the DT nodes looking for the panels
the kernel supports for OMAP. For each found node, the 'compatible'
string is prepended with "omapdss,", i.e. "sony,acx565akm" becomes
"omapdss,sony,acx565akm". The OMAP display drivers all have "omapdss,"
at the beginning of their compatible field.

This allows us to have generic DT data, but OMAP specific display
drivers.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>