This value was being used uninitialized, resulting in predictable issues
on systems with memory-mapped UART registers.
A case could be made that memmap_bus should be declared in a header
rather than being declared in each .c file which needs to refer to it,
but that's a broader style question.
This commit unbreaks hw.uart.console="mm:..." on ARM64.
Submitted by: Greg V
0624eddcff