[ipmi] Generate bifurcation for item without associations

This CL make IPMI dynamic bifurcation to generate bifurcation for device
without association.

This is the case for devices like Y-cable on
RP. Only the primary side of y-cable has downstream, the other
side(P1) doesn't have dowmstream. Without this fix, P1 side of the
Y-cable doesn't have the right bifurcation.

Tested:
All sides of Y-cable has 8x8 bifurcation
root@lcsqli-j6-nfd01:~# for slot in 1 2 5 6; do echo -n "Slot $slot Bifurcation: "; ipmitool raw 0x2e 0x32 0x79 0x2b 0x00 0x0f $slot; done
Slot 1 Bifurcation:  79 2b 00 0f 02 08 08
Slot 2 Bifurcation:  79 2b 00 0f 02 08 08
Slot 5 Bifurcation:  79 2b 00 0f 02 08 08
Slot 6 Bifurcation:  79 2b 00 0f 02 08 08

Google-Bug-Id: 483873442
Change-Id: I17fa5095a29cd219c3d5dee5f28dc8d835e35c52
Signed-off-by: Tao Ke <taok@google.com>