sunxi: video: Add a hpd_delay parameter to configure hpd delay

In some extreme cases it may be necessary to wait 1.5 seconds or more for a hpd
signal to show up (and be able to read edid info), but we do not want to
penalize all headless boots with an extra second boot delay, so add a hpd_delay
parameter which can be set through the video-mode env. variable.

While at it raise the default from 300ms to 500ms as 300 may very well be too
low in many cases.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>