Update OpenBIOS images

Update PPC, Sparc32 and Sparc64 OpenBIOS images to r821.
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

pc-bios/README

@@ -15,7 +15,7 @@
   firmware implementation. The goal is to implement a 100% IEEE
   1275-1994 (referred to as Open Firmware) compliant firmware.
   The included image for PowerPC (for 32 and 64 bit PPC CPUs), Sparc32
-  and Sparc64 are built from OpenBIOS SVN revision 795.
+  and Sparc64 are built from OpenBIOS SVN revision 821.
 
 - The PXE roms come from Rom-o-Matic gPXE 0.9.9 with BANNER_TIMEOUT=0