powerpc/configs: Add Skiroot defconfig

This configuration is used by the OpenPower firmware for it's
Linux-as-bootloader implementation. Also known as the Petitboot
kernel, this configuration broke in 4.12 (CPU_HOTPLUG=n), so add it to
the upstream tree in order to get better coverage.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>