powerpc/oprofile: G4 oprofile has variable number of counters

For ppc750 processors which use 4 performance counters instead of the
6 G4 uses but otherwise is compatible with G4.

Signed-off-by: Octavian Purdila <opurdila@ixiacom.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>