[coreboot] Different register numbers to program subsystem ids

[Josef Kellermann]

The default 'pci_set_subsystem' sets the subvendor/device id register #0x2c. In fact for a few devices in the RS690 the register is 0x4c respectively 0x50. Moved the default set_subsystem implementation to chipset specific.

Signed-off-by: Josef Kellermann