ARM: defconfig: enable memory prealloc for mdmfermium

enable memory prealloc config flag to allocate memory
for the WLAN driver module in mdmfermium target.

Change-Id: Id117eb3edba59969fcb25c0aa7788f6d2101d064
Signed-off-by: Sarada Prasanna Garnayak <sgarna@codeaurora.org>