powerpc/pmac32: enable serial options by default in defconfig

Serial is a critical feature for logging and debuging, and the other
architectures enable serial by default.

Let's enable CONFIG_SERIAL_PMACZILOG and CONFIG_SERIAL_PMACZILOG_CONSOLE
by default.

Signed-off-by: Yuan Tan <tanyuan@tinylab.org>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/bb7b5f9958b3e3a20f6573ff7ce7c5dc566e7e32.1690982937.git.tanyuan@tinylab.org