powerpc/64: Move HAVE_CONTEXT_TRACKING from pseries to common Kconfig

We added support for HAVE_CONTEXT_TRACKING, but placed the option inside
PPC_PSERIES.

This has the undesirable effect that NO_HZ_FULL can be enabled on a
kernel with both powernv and pseries support, but cannot on a kernel
with powernv only support.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>