[POWERPC] powerpc: Divorce CPU_FTR_CTRL from CPU_FTR_PPCAS_ARCH_V2_BASE

The performance monitor implementation (including CTRL register behaviour)
is just included in PPC v2 as an example, it's not truly part of the base.

It's actually a somewhat misleading feature, but I'll leave that be for
now: The presence of the register is not what the feature bit is used
for, but instead it's used to determine if it contains the runlatch
bit for idle reporting of the performance monitor. For alternative
implementations, the register might still exist but the bit might have
different meaning (or no meaning at all).

For now, split it off and don't include it in CPU_FTR_PPCAS_ARCH_V2_BASE.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>