powerpc/64s: Explicitly add vector features to CPU_FTRS_POSSIBLE

ALTIVEC and VSX features are not added by to default to the POWERx CPU
feature sets because they are intended to be enabled by firmware.
Currently they end up in CPU_FTRS_POSSIBLE due to their inclusion in
other the set for other CPUs, eg. PPC970.

But they should be added individually to the CPU_FTRS_POSSIBLE set,
because if we reduce the set of CPUs that are built-for they may
disappear from the possible mask.

It already contains CPU_FTR_VSX, so add ALTIVEC. The _COMP features
should be used because they won't be present if compiled out.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Add detail to change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>