mips: teach the malta platform about extended memory.

Extended memory here is "physical memory above 256MB".
"memsize" in the environment only grows to 256MB; "ememsize" is the entire
memory range. Extended memory shows up at physical address 0x90000000.

This allows for malta64 VMs to be created with > 256MB RAM, all the way
up to 2GB RAM.

Tested:

• qemu-devel package; qemu-system-mips64 -m 2048 (and -m 256 to test the no-ememsize case.)

TODO:

• testing mips32 with > 256MB RAM.

Reviewed by: imp