soc/amd/common/data_fabric/domain: write _SEG method in SSDT

As a preparation for the multi PCI segment group support, use
acpigen_write_BBN to generate the _SEG method that returns the segment
group number of the PCI root. Until the multi PCI segment group support
is enabled in coreboot, it will always return 0.

Signed-off-by: Felix Held <felix-coreboot@felixheld.de>
Change-Id: I2a812dcc564c5319385e9ad482d29b2984a71b8a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/79924
Reviewed-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/soc/amd/common/block/data_fabric/domain.c

@@ -287,6 +287,7 @@ void amd_pci_domain_fill_ssdt(const struct device *domain)
 
 	acpigen_write_resourcetemplate_footer();
 
+	acpigen_write_SEG(0);
 	acpigen_write_BBN(domain->link_list->secondary);
 
 	/* Scope */