Initialize uart_bus_space_mem.

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