powerpc/configs: Disable legacy ptys on microwatt defconfig

We shouldn't need legacy ptys, and disabling the option improves boot
time by about 0.5 seconds.

Signed-off-by: Anton Blanchard <anton@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210805112005.3cb1f412@kryten.localdomain