sparc64 address-congruence property

The Machine Description (MD) property "address-congruence-offset" is
optional. According to the MD specification the value is assumed 0UL when
not present. This caused early boot failure on T5.

Signed-off-by: Bob Picco <bob.picco@oracle.com>
CC: sparclinux@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>