powerpc/85xx: Don't use generic timebase sync on 64-bit

85xx currently uses the generic timebase sync mechanism when
CONFIG_KEXEC is enabled, because 32-bit 85xx kexec support does a hard
reset of each core.  64-bit 85xx kexec does not do this, so we neither
need nor want this (nor is the generic timebase sync code built on
ppc64).

FWIW, I don't like the fact that the hard reset is done on 32-bit
kexec, and I especially don't like the timebase sync being triggered
only on the presence of CONFIG_KEXEC rather than actually booting in
that environment, but that's beyond the scope of this patch...

Signed-off-by: Scott Wood <scottwood@freescale.com>