powerpc: Make corenet64_defconfig a bit more useful

CONFIG_BLK_DEV_SD, SR, ... are needed for pretty much any SATA or USB
storage device (corenet32_defconfig has them) and modern any with
systemd needs the CGROUPS gunk.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Scott Wood <scottwood@freescale.com>