powerpc: Little endian should depend on PPC_BOOK3S_64

The only little endian configuration we support is ppc64le, all other
configurations are big endian.

So we should only offer a choice of endian if we're building for 64-bit
Book3S, ie. PPC_BOOK3S_64.

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