ARM: enable vmalloc saving

For some targets that have less vmalloc space this can be increased by
enabling config ENABLE_VMALLOC_SAVING. With this config we can reclaim
virtual mappings which remains unused because of non hlos carveout
reservations in lowmem.

Select the default method of reclaiming virtual memory as vmalloc saving.

Change-Id: I05bbae3b554be54f74b740404dda0cdfa89084bd
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>