libpldm: implement encode/decode APIs for GetPDR

This commit implements the encode response and decode request of
GetPDR command which is defined in PLDM Platform spec DSP0248_1.1.1
This enables the PLDM responder to receive the request and process
the response packet.

Signed-off-by: Sampa Misra <sampmisr@in.ibm.com>
Change-Id: I9c17e0a4dd4e02b3c8b50eb55958c687c988f421