powerpc/iseries: Fix early init access to lppaca

The combination of commit

8154c5d2 and
93c22703



Broke boot on iSeries.

The problem is that iSeries very early boot code, which generates
the device-tree and runs before our normal early initializations
does need access the lppaca's very early, before the PACA array is
initialized, and in fact even before the boot PACA has been
initialized (it contains all 0's at this stage).

However, the first patch above makes that code use the new
llpaca_of(cpu) accessor, which itself is changed by the second patch to
use the PACA array.

We fix that by reverting iSeries to directly dereferencing the array. In
addition, we fix all iterators in the iSeries code to always skip CPU
whose number is above 63 which is the maximum size of that array and
the maximum number of supported CPUs on these machines.

Additionally, we make sure the boot_paca is properly initialized
in our early startup code.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>