powerpc: Remove CLASSIC_PPC

We have a strange #define in cputable.h called CLASSIC_PPC.

Although it is defined for 32 & 64bit, it's only used for 32bit and
it's basically a duplicate of CONFIG_PPC_BOOK3S_32, so let's use
the latter.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>