pseries: Update SLOF firmware image

The only change that SLOF does not rely on QEMU providing an RTAS blob
and provides one itself:
https://git.qemu.org/?p=SLOF.git;a=commitdiff;h=5e4ed1fd0f39e



Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David 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-20190703.
+  built from git tag qemu-slof-20190719.
 
 - sgabios (the Serial Graphics Adapter option ROM) provides a means for
   legacy x86 software to communicate with an attached serial console as

SLOF @ 7bfe584e

-Subproject commit ba1ab360eebe6338bb8d7d83a9220ccf7e213af3
+Subproject commit 7bfe584e321946771692711ff83ad2b5850daca7