drivers/cpufreq: change CONFIG_6xx to CONFIG_PPC_BOOK3S_32

Today, powerpc has three CONFIG labels which means exactly the same:
- CONFIG_6xx
- CONFIG_PPC_BOOK3S_32
- CONFIG_PPC_STD_MMU_32

By consistency with PPC64, CONFIG_PPC_BOOK3S_32 is the preferred one.
Using a label with includes _PPC_ also makes it clearer that it is
linked to powerpc.

In preparation of the removal of CONFIG_6xx, this patch replaces it
by CONFIG_PPC_BOOK3S_32

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>