Write an ABI document capturing required behaviours for the BMC and host firmware

[amboar]

We have requirements on what the BMC should initialise and the system configuration that the host assumes for ASPEED BMC-based machines (OpenPOWER).

1. The VUART is used and is present at 0x3f8 in the LPC IO space, using SerIRQ 4
2. The IPMI BT interface is used and is present at 0xe4 in the LPC IO space, using SerIRQ 10
3. The BMC shall perform VGA handover upon detecting the presence of the VGA password in the SCU VGA scratch registers.