sunxi: ahci: Add a delay after enabling target power

If the target power is connected through a gpio, then give the target some
time to power up before continuing with ahci / sata probing, this avoids
link timeouts, without penalizing other boards where there is no target
power gpio.

Why 500 ms ? I started with 200, that was not enough, then I went to 500 which
worked, lowering it to 350 broke things again, so 500 seems the minimum my
vertex2 needs to be ready to get probed.

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