powerpc: Make sure that we alays include CONFIG_BINFMT_ELF

Our kernel is not much good without BINFMT_ELF and this fixes a build
warning on 64 bit allnoconfig builds:

warning: (COMPAT) selects COMPAT_BINFMT_ELF which has unmet direct dependencies (COMPAT && BINFMT_ELF)

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>