util: Make the device file name used by map_m5_mem overridable.

util: Make the device file name used by map_m5_mem overridable.

The name this function uses is now exposed as a global variable called
m5_mmap_dev which can be changed at run time.

This would be useful if using a non-standard location for /dev/mem, or
for testing where we might want to use a totally different device.

Change-Id: I5e7ac106c3e4e0555c99af2a7a0aca8171534451
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27556
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Pouya Fotouhi <pfotouhi@ucdavis.edu>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>