pseries: Update SLOF firmware image

The only change here is moving the decision about rtas-size
to QEMU.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

pc-bios/README

@@ -14,7 +14,7 @@
 - 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-20200312.
+  built from git tag qemu-slof-20200317.
 
 - sgabios (the Serial Graphics Adapter option ROM) provides a means for
   legacy x86 software to communicate with an attached serial console as

SLOF @ ab6984f5

-Subproject commit 8d4033d0d2e5b434fe14a81d5192cdb1911469e4
+Subproject commit ab6984f5a6d054e1f634dda855b32e5357111974