powerpc: Kconfig: remove BE-only platforms from LE kernel build

Currently, little endian is only supported on powernv and pseries,
however, Kconfigs still allow us to include other platforms in a LE
kernel, this may result in space wasting or even build error if some
BE-only platforms always assume they are built for a BE kernel. So just
modify the Kconfigs of BE-only platforms to remove them from being built
for a LE kernel.

For 32bit only platforms, nothing needs to be done, because
CPU_LITTLE_ENDIAN depends on PPC64. For 64bit supported platforms, add
CPU_BIG_ENDIAN to dependencies explicitly, so that these platforms will
be disabled for LE [Suggested-by: Cédric Le Goater <clg@fr.ibm.com&gt;].>

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>