oem_ibm: Tie up the pvm_system_name with the AssetTag

- This commit would add the support for binding both the
  pvm_system_name bios attribute to redfish AssetTag.

Tested :

1. Change the bios attribute & make sure the AssetTag is updated.
2. Change the AssetTag and make sure the bios attribute is updated.

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: Ifef3abb56a6826304ce292c5ef1609f0740d5bdd