defconfig: apq8084: Enable CONFIG_WCNSS_MEM_PRE_ALLOC flag

Enable this feature so that it will reserve chunks of memory at boot
time and WLAN driver will be able to use this pre-allocated memory (when
available) for large memory allocations. This will prevent WLAN driver
failures because of the un-availability of large size slabs.

Change-Id: I1f69af7fcb27df8094bac61cd11b47e251dff520
CRs-fixed: 662637
Signed-off-by: Yue Ma <yuem@codeaurora.org>