pldm: Document BIOS support

The BIOS attributes supported by OpenBMC systems are added in JSON
files, adhering to the specifications outlined in the PLDM BIOS
Specification [1]. Since a single BIOS attribute may have different
values for different variants of systems, creating different json files
for each of the system type supported.  To enable this optional feature
within PLDM, creating meson option system-specific-bios-json.

System specific bios attributes support in PLDM is added in below
commits [2], [3], [4]

[1]: https://www.dmtf.org/sites/default/files/standards/documents/DSP0247_1.0.0.pdf
[2]: https://github.com/openbmc/pldm/tree/963c0fdf1aef305f266e00b2321fc244f89d0fa7
[3]: https://github.com/openbmc/pldm/tree/62dd8ff2f106c0eb80e8c1e79adfa99e093d1cf6
[4]: https://github.com/openbmc/pldm/tree/46f352ed2426368185f3bd36fd6bf88f15fa5111



Change-Id: Ieffdca541d1ff8489911b02c178f85d3b1b84a5c
Signed-off-by: Archana Kakani <archana.kakani@ibm.com>