powerpc: change CONFIG_PPC_STD_MMU to CONFIG_PPC_BOOK3S

Today we have:

config PPC_BOOK3S
        def_bool y
        depends on PPC_BOOK3S_32 || PPC_BOOK3S_64

config PPC_STD_MMU
        def_bool y
        depends on PPC_BOOK3S

PPC_STD_MMU is therefore redundant with PPC_BOOK3S. Lets remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>