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

sys/dev/uart/uart_cpu_arm64.c

@@ -65,8 +65,9 @@ __FBSDID("$FreeBSD$");
 /*
  * UART console routines.
  */
+extern struct bus_space memmap_bus;
 bus_space_tag_t uart_bus_space_io;
-bus_space_tag_t uart_bus_space_mem;
+bus_space_tag_t uart_bus_space_mem = &memmap_bus;
 
 int
 uart_cpu_eqres(struct uart_bas *b1, struct uart_bas *b2)