powerpc: Reduce differences between pseries and ppc64 defconfigs

The pseries and ppc64 defconfigs have drifted apart over the years. Reduce
some of the differences while still keeping the idea that the ppc64 defconfig
is cross platform but enables fewer features than pseries, eg NR_CPUS is
lower.

Also enable a number of common adapters as modules.

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