drm/nouveau/falcon: support for EMEM

On SEC, DMEM is unaccessible by the CPU when the falcon is running in LS
mode. This makes communication with the firmware using DMEM impossible.

For this purpose, a new kind of memory (EMEM) has been added. It works
similarly to DMEM, with the difference that its address space starts at
0x1000000. For this reason, it makes sense to treat it like a special
case of DMEM.

Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>