powerpc: Add ppc64le and ppc64_book3e allmodconfig targets

Similarly as we just did for 32-bit, add phony targets for generating
a little endian and Book3E allmodconfig. These aren't covered by the
regular allmodconfig, which is big endian and Book3S due to the way
the Kconfig symbols are structured.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>