powerpc/configs: Add a powernv_be_defconfig

Although pretty much everyone using powernv is running little endian,
we should still test we can build for big endian. So add a
powernv_be_defconfig, which is autogenerated by flipping the endian
symbol in powernv_defconfig.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Cyril Bur <cyrilbur@gmail.com>