Update PowerPC OpenBIOS image to r721

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

pc-bios/README

@@ -14,8 +14,9 @@
 - OpenBIOS (http://www.openbios.org/) is a free (GPL v2) portable
   firmware implementation. The goal is to implement a 100% IEEE
   1275-1994 (referred to as Open Firmware) compliant firmware.
-  The included images for PowerPC (for 32 and 64 bit PPC CPUs),
-  Sparc32 and Sparc64 are built from OpenBIOS SVN revision 683.
+  The included image for PowerPC (for 32 and 64 bit PPC CPUs) is
+  built from OpenBIOS SVN revision 721, the Sparc32 and Sparc64 ones
+  are built from OpenBIOS SVN revision 683.
 
 - The PXE roms come from Rom-o-Matic gPXE 0.9.9 with BANNER_TIMEOUT=0