powerpc/configs: Add ppc64le_allnoconfig target

Add a phony target for ppc64le_allnoconfig, which tests some
combinations of CONFIG symbols that aren't covered by any of our
defconfigs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201125031551.2112715-1-mpe@ellerman.id.au