sunxi: Reduce Orangepi PC RAM speed to 624 MHz

There are some reports of stability issues at 672 MHz, see:
http://linux-sunxi.org/Orange_Pi_PC#DRAM_clock_speed_limit



So reduce the DRAM speed to 624MHz which seems to be reliable everywhere.

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