On M8 chips, use a max_phys_bits value of 51.

Also, M8 supports VA bits up to 54 bits. However, for now
restrict VA bits to 53 due to 4-level pagetable limitation.
Signed-off-by: Vijay Kumar <vijay.ac.kumar@oracle.com>
Reviewed-by: Bob Picco <bob.picco@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>