powerpc/64s: Default to 64K pages for 64 bit book3s

For 64-bit book3s the default should be 64K as that's what modern CPUs
are designed for.

The following defconfigs already set CONFIG_PPC_64K_PAGES:

 cell_defconfig
 pasemi_defconfig
 powernv_defconfig
 ppc64_defconfig
 pseries_defconfig
 skiroot_defconfig

The have the option removed from the defconfig, as it is now the
default.

The defconfigs that now need to set CONFIG_PPC_4K_PAGES to maintain
their existing behaviour are:

 g5_defconfig
 maple_defconfig
 microwatt_defconfig
 ps3_defconfig

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
BugLink: https://github.com/linuxppc/issues/issues/109
Link: https://lore.kernel.org/r/20211015001649.45591-1-joel@jms.id.au