defconfig: msm8916: disable CONFIG_HIGHMEM for 512MB platforms

CONFIG_HIGHMEM has some performance impact when enabled, even
for platforms with only lowmem (0 highmem). Hence disable
CONFIG_HIGHMEM option for 512MB based msm8916
platforms.

CRs-fixed: 640379
Change-Id: I0b2dcb35c3868cb1ba506821ed8fad158be4c410
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>