powerpc: allyesconfig should not select CONFIG_CPU_LITTLE_ENDIAN

Stephen reported a failure in an allyesconfig build.
CONFIG_CPU_LITTLE_ENDIAN=y gets set but his toolchain is not
new enough to support little endian. We really want to
default to a big endian build; Ben suggested using a choice
which defaults to CPU_BIG_ENDIAN.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>