defconfig: Enable config to allow vmalloc savings

Enable the config ENABLE_VMALLOC_SAVING to reclaim
virtual mappings which remains unused because of
non hlos carveout reservations in lowmem.

Change-Id: I795b49d1dbc758b652f53b7f249073599c3eae62
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>