sparc64: Increase size of boot string to 1024 bytes

This is the longest boot string that silo supports.

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