pseries: Update SLOF firmware image

This aims v4.2 and fixes:
1. full FDT rendering;
2. gcc9 -Waddress-of-packed-member.
Signed-off-by: NAlexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

pc-bios/README

@@ -17,7 +17,7 @@
 - SLOF (Slimline Open Firmware) is a free IEEE 1275 Open Firmware
   implementation for certain IBM POWER hardware.  The sources are at
   https://github.com/aik/SLOF, and the image currently in qemu is
-  built from git tag qemu-slof-20190911.
+  built from git tag qemu-slof-20191022.
 
 - sgabios (the Serial Graphics Adapter option ROM) provides a means for
   legacy x86 software to communicate with an attached serial console as

SLOF @ 899d9883

-Subproject commit bcc3c4e5c21a015f4680894c4ec978a90d4a2d69
+Subproject commit 899d98836513bb3d6a4f4e48ef7cee887ee5f57b