pseries: Update SLOF firmware image

This mainly fixes virtio-serial with and without
enabled iommu-platform.

The full list of changes is:

Alexey Kardashevskiy (3):
      llfw: Fix debug printf warnings
      virtio-serial: Close device completely
      version: update to 20200312

Cédric Le Goater (1):
      virtio: Fix typo in virtio_serial_init()

Greg Kurz (2):
      virtio-serial: Don't override some words
      virtio-serial: Rework shutdown sequence

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-20200221.
+  built from git tag qemu-slof-20200312.
 
 - sgabios (the Serial Graphics Adapter option ROM) provides a means for
   legacy x86 software to communicate with an attached serial console as