src

When no legacy serial is found, we may be looking at a non-legacy mmio
serial device mapping, in which case the efi_devpath_name() for name
ConOutDev looks like this:

    VenHw(XXXXXXXX-XXXX-XXXX-XXXX-XXXXXXXXXXXX,0090DCFE00000000)/Uart(115200,8,N,1)/VenVt100()

Which should tell the kernel to attach a console to 0xfedc9000
(little endian 64 bit value).  The value is stored behind the
VENDOR_DEVICE_PATH struct as a byte stream hence we need to check
if said address is appended behind the node.  Also enforce use for
uart by requiring the console speed read from the same device.

There is no scientific process for "rs:2" derivation, but evidence would
indicate that this is the correct setting for existing MMIO EFI consoles.

See also: http://bsdimp.blogspot.com/2018/07/how-to-get-memory-mapped-serial-console.html