configs/qemu_ppc_mac99_defconfig: add usual comments for Kconfig symbols

This defconfig was generated by savedefconfig but we usually
use a manually modified defconfig to add some comments for
Kconfig symbols.

No content change intended.

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Joel Stanley <joel@jms.id.au>
Acked-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>