sunxi: reduce Orange Pi Zero DRAM clock speed

sunxi: reduce Orange Pi Zero DRAM clock speed

Using `fel-boot-lima-memtester-on-orange-pi-pc 672` on an Orange Pi
without heatsink results in the following error after a few minutes:
  WRITE FAILURE: 0x00200000 != 0xffdfffff at offset 0x0137f47c (bitflip).

Also, the constructor repository (github/orangepi-xunlong) seems to
contain that 624 Mhz clock speed in its u-boot fork. It may be that 672
Mhz is the advertized overclocked speed.

According to http://linux-sunxi.org/Orange_Pi_PC#DRAM_clock_speed_limit
it may be worth decreasing that value with other Orange Pi boards.

See also e7d6aa0b74b7f4d08ee68da8a586c76c761348e2.

Signed-off-by: Clément Bœsch <u@pkh.me>
[Add s-o-b line]
Signed-off-by: Jagan Teki <jagan@openedev.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>