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

In some 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 boots with an extra second boot delay, so add a hpd_delay parameter which
can be set through the video-mode env. variable.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>