s390: add initial 64-bit restart PSW

To be able to start a kernel image loaded into memory with a
PSW restart, place a 64-bit restart PSW at 0x1a0 in absolute
lowcore.
Suggested-by: Dominik Klein <dominik.klein@linux.ibm.com>
Tested-by: Dominik Klein <dominik.klein@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/boot/head.S

@@ -60,6 +60,9 @@ __HEAD
 	.long	0x02000690,0x60000050
 	.long	0x020006e0,0x20000050
 
+	.org	0x1a0
+	.quad	0,iplstart
+
 	.org	0x200
 
 #