sisfb: add RAM type detection for XGI Z9

Detect the XGI Z9 RAM type as "documented" by the XGI's xgifb driver.

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