fru: server 3.0: Set bifurcation without a bus

Summary:
For platform 30, there are some root complex configurations that aren't
related to a PCIe slot. The host is still expecting the BMC to provide
this information for these utility clusters even though these values
won't ever change.

Allow configuration entries without a specified I2C bus.

Tested:
Built successfully.

Google-Bug-Id: 435526024
Change-Id: Ieba0998d18764dd28f76f7708a65fe9e951ee418
Signed-off-by: Kyle Nieman <kyle.nieman@fii-na.corp-partner.google.com>