defconfig: mdmcalifornium: Enable prealloc feature for WLAN

Prealloc feature reserves a few chunks of memory during system boots
which can help WLAN driver to be loaded successfully when memory is low
or fragmented after system long run.

Change-Id: I29074370595631e470a9f20dd5cb09d897aef7db
Signed-off-by: Yue Ma <yuem@codeaurora.org>