device/main/linux-postmarketos-allwinner: Enabe LRU_GEN by default (MR 3686)

MGLRU seeks to address Linux's poor page reclamation behavior with this new high performance LRU implementation to over-commit memory.
MGLRU has shown to be beneficial in a variety of benchmarks especially on memory constrained systems.

Some benchmarks:
- https://www.phoronix.com/news/Linux-MGLRU-v9-Promising
- https://www.phoronix.com/news/MGLRU-Reaches-mm-stable

[ci:skip-build]: Already built successfully in CI