sunxi: Update DRAM clock for Olimex A20 boards

Originally dram clock was set to 480MHz, but this behaves
unstable. To improve stability the clock is reduced to 384MHz

Signed-off-by: Stefan Mavrodiev <stefan.mavrodiev@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>