PPC: mac_nvram: Split NVRAM into OF and OSX parts

Mac OS X (at least with -M mac99) searches for a valid NVRAM partition
of a special Apple type. If it can't find that partition in the first
half of NVRAM, it will look at the second half.

There are a few implications from this. The first is that we need to
split NVRAM into 2 halves - one for Open Firmware use, the other one for
Mac OS X. Without this split Mac OS X will just loop endlessly over the
second half trying to find a partition.

The other implication is that we should provide a specially crafted Mac
OS X compatible NVRAM partition on the second half that Mac OS X can
happily use as it sees fit.

Signed-off-by: Alexander Graf <agraf@suse.de>
