powerpc: Introduce CONFIG_PPC_BOOK3S

This patch introduce a new Kconfig option, CONFIG_PPC_BOOK3S
that represents processors that are compliant with the "classic"
(aka "server") variant of the PowerPC architecture.

It replaces CONFIG_6xx on 32-bit (though the symbol is still
defined for compatibility) and encompass all currently supported
64-bit processors.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>