bitbake.conf: Adds bitbake qemu option for ppc e6500 & ppc e6500-64b.

This patch adds the bitbake qemu option for the ppc e6500 &
ppc e6500-64b architectures.

Signed-off-by: Valentin Cobelea <valentin.cobelea@enea.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>