ppc/pnv: update skiboot to v6.4

Currently we fail to boot a qemu powernv machine with a Power9
processor:

 PLAT: Detected generic platform
 PLAT: Detected BMC platform generic
 CPU: All 1 processors called in...
 CHIPTOD: Unknown TOD type !
 CHIPTOD: Failed ChipTOD detection !
 Aborting!

With v6.4 we can boot both a Power8 and Power9 powernv machine.

Built from submodule with powerpc64le-linux-gnu-gcc (Debian 8.3.0-2).

Signed-off-by: Joel Stanley <[email protected]>
Message-Id: <[email protected]>
Reviewed-by: Cédric Le Goater <[email protected]>
Signed-off-by: David Gibson <[email protected]>

Author: shenki