powerpc: Fix setting of oprofile cpu type

commit 2657dd4e introduced a
bug where we would now always override the "real" oprofile CPU
type with the "compatible" one provided by a pseudo-PVR in the
device-tree which is incorrect and breaks oprofile on all current
configs since the "compatible" ones aren't yet recognized.

This fixes it.

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