Enter 64-bit mode as early as possible in the 64-bit PowerPC boot sequence.

Enter 64-bit mode as early as possible in the 64-bit PowerPC boot sequence.
Most of the effect of setting MSR[SF] is that the CPU will stop ignoring
the high 32 bits of registers containing addresses in load/store
instructions. As such, the kernel was setting it only when it began to
need access to high memory. MSR[SF] also affects the operation of some
conditional instructions, however, and so setting it at late times could
subtly break code at very early times. This fixes use of the FDT mode in
loader, and FDT boot more generally, on 64-bit PowerPC systems.

Hardware provided by: IBM LTC
Approved by: re (kib)