sparc64: Add 16GB hugepage support

Adds support for 16GB hugepage size. To use this page size
use kernel parameters as:

default_hugepagesz=16G hugepagesz=16G hugepages=10

Testing:

Tested with the stream benchmark which allocates 48G of
arrays backed by 16G hugepages and does RW operation on
them in parallel.

Orabug: 25362942

Cc: Anthony Yznaga <anthony.yznaga@oracle.com>
Reviewed-by: Bob Picco <bob.picco@oracle.com>
Signed-off-by: Nitin Gupta <nitin.m.gupta@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>