sisfb: add support for XGI Z9 DDR2 POST

Add support for ZGI Z9 DDR2 POST. The init sequence is from XGI's
xgifb driver.

Tested with ARM board using a PCI card with XGI Z9s and 32 MB DDR2
memory. After a cold reset the POST succeeds.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>