powerpc/44x: Don't support 440 when CONFIG_PPC_47x is set

As stated in platform/44x/Kconfig, CONFIG_PPC_47x is not
compatible with 440 and 460 variants.

This is confirmed in asm/cache.h as L1_CACHE_SHIFT is different
for 47x, meaning a kernel built for 47x will not run correctly
on a 440.

In cputable, opt out all 440 and 460 variants when CONFIG_PPC_47x
is set. Also add a default match dedicated to 470.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/822833ce3dc10634339818f7d1ab616edf63b0c6.1603041883.git.christophe.leroy@csgroup.eu