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>

arch/powerpc/platforms/Kconfig.cputype

@@ -431,6 +431,7 @@ config CPU_BIG_ENDIAN
 
 config CPU_LITTLE_ENDIAN
 	bool "Build little endian kernel"
+	depends on PPC_BOOK3S_64
 	select PPC64_BOOT_WRAPPER
 	help
 	  Build a little endian kernel.