Replying to myself after more experimentation:

[The other possible thing to do is to use the siimage driver in
drivers/ide, but this one stopped working with 2.6.8 and further. The
siimage driver works quite good with 2.6.7, so this is another proof for no
hardware problems]

Ok, support for the deprected siimage driver has to be explicitly enabled  
unter drivers/ide (CONFIG_BLK_DEV_IDE_SATA). Then it recognizes the chipset 
and seems to work so far.

The only annoyance is that the driver hangs for some seconds probing the 
second port (which is unused):

SiI3112 Serial ATA: IDE controller at PCI slot 0000:00:0a.0
PCI: Found IRQ 5 for device 0000:00:0a.0
SiI3112 Serial ATA: chipset revision 1
SiI3112 Serial ATA: 100% native mode on irq 5
    ide2: MMIO-DMA , BIOS settings: hde:DMA, hdf:DMA
    ide3: MMIO-DMA , BIOS settings: hdg:pio, hdh:pio
Probing IDE interface ide2...
hde: SAMSUNG SP1614C, ATA DISK drive
ide2 at 0xd0800080-0xd0800087,0xd080008a on irq 5
Probing IDE interface ide3...
hdg: no response (status = 0xfe)

But one can live with that.

Greetings
Bausi