pseries: Update SLOF firmware image

pseries: Update SLOF firmware image

This includes spapr-vio and usb-storage fixes, phandles fix for NVLink2
pass through support and other compile improvements.

The full list of changes is:
  * vio-vscsi: Support multiple channels / buses
  * board-qemu/slof/vio-vscsi: Scan up to 64 SCSI IDs
  * usb/storage: Implement block write support
  * usb/storage: Invert the logic of the IF-statements
  * fdt: Fix phandles for NVLink/NVLink2
  * fdt: Factor out code to replace a phandle in place
  * pci: use appropriate base class ids
  * Makefile: Set a proper DRIVER_NAME when building from a git tree
  * romfs/tools: Silence more compiler warnings with GCC 8.1
  * romfs/tools: Silence GCC 8.1 compiler warning with FLASHFS_MAGIC
  * romfs/tools: Remove superfluous union around the rom header struct
  * make.rules: Compile SLOF with -fno-asynchronous-unwind-tables

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