Increase space for storing DID numbers

[mpernambuco]

Context

DID numbers are limited resources as they are stored within 4 bits of pma_entry.start.
We are close to run out of space for storing new DIDs.
The last available DID number will be soon taken by the new shadow uarch range.
We want to make room for new DID numbers.

Possible solutions

Utilize the unused 12 least significant bits of pma_entry.length to store DID numbers.
Move the DID number from pma_entry.start to 5 or more bits of pma_entry.length.