pmb/config: add required option for Waydroid support

CONFIG_PSI is required for Waydroid Android containers, it's used by a userspace OOM killer within Android. See https://gitlab.com/postmarketOS/pmaports/-/merge_requests/2497 for more details.

Marking this draft for now as it might have a performance impact we don't want and thus needs testing before we decide on this permanently.