Reduce multiple oem_ibm entry points in pldmd

Abstract the custom method of `OEM-IBM` into the oem-ibm.hpp file
to prevent the continuous increase of custom code and reduce multiple
`OEM-IBM` entry points in pldmd.

Tested: enabled oem-ibm and built pldm successfully.

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: Ieddb8d12281553e70bdb1c333bd29425c9d14fb0