pseries: Update SLOF firmware image to qemu-slof-20150813

The changes are:
1. GPT support;
2. Much faster VGA support.

The full changelog is:
  > Add missing half word access case to _FASTRMOVE and _FASTMOVE
  > Remove unused RMOVE64 stub
  > fbuffer: Implement RFILL as an accelerated primitive
  > fbuffer: Implement MRMOVE as an accelerated primitive
  > fbuffer: Precalculate line length in bytes
  > terminal: Disable the terminal-write trace by default
  > boot: remove trailing ":" in the bootpath
  > ci: implement boot client interface
  > boot: bootpath should be complete device path
  > fbuffer: Use a smaller cursor
  > fbuffer: Improve invert-region helper
  > usb-hid: Caps is not always shift
  > cas: Increase FDT buffer size to accomodate larger ibm, cas node properties
  > README: Update with patch submittion note
  > disk-label: add support for booting from GPT FAT partition
  > disk-label: introduce helper to check fat filesystem
  > introduce 8-byte LE helpers
  > disk-label: simplify gpt-prep-partition? routine
  > fbuffer: introduce the invert-region-x helper
  > fbuffer: introduce the invert-region helper
  > fbuffer: simplify address computations in fb8-toggle-cursor

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