defconfig: Enable CONFIG_VMSPLIT_3G_OPT

sdxlemur will have at most 1GB of DDR, so allow all of it to be
mapped as lowmem instead of enabling CONFIG_HIGHMEM.

Change-Id: I89148d18e0be0c1d99795b7f49184bb5421af0f1
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>