mips_malta: do not raise exceptions when accessing invalid memory

Since commit c658b94f6e8c206c59d02aa6fbac285b86b53d2c, MIPS raises
exceptions when accessing invalid memory. This is not the correct
behaviour for MIPS Malta Core LV, as the GT-64120A system controller
just ignore undecoded access. This feature is used by the Linux kernel
to probe for some devices.

Emulate the correct behaviour in QEMU by adding an empty slot covering
the entire memory space decoded by the GT-64120A.

Tested-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
