dt: Add option for dpi without DE and PCLK (for VGA666)

VGA666 doesn't use the DE or PCLK signals, therefore there is
no point in claiming their use. It's also then possible to
use GPIOs 0&1 for DDC to read the EDID from the display.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.com>