mgmt/osdp: Add support for OSDP in PD mode of operation

Open Supervised Device Protocol (OSDP) describes the communication
protocol for interfacing one or more Peripheral Devices (PD) to a
Control Panel (CP). The PDs are slave devices that waits for commands
from a CP. The communication happens over a RS485 multi-drop connection
with specification for a secure channel communication.

This patch adds initial support for OSDP in PD mode without secure
channel.

Signed-off-by: Siddharth Chandrasekaran <siddharth@embedjournal.com>