powerpc/configs/microwatt: Enable options for systemd

When booting with systemd these options are required.

This increases the image by about 50KB, or 2%.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210826122653.3236867-4-joel@jms.id.au