defconfig: msm8994: Enable marking free pages as readonly option

FREE_PAGES_RDONLY debug feature helps to mark the pages
as read only after they are freed and helps to catch the
corruption at source in some scenarios.

Change-Id: I72a0bb7b2e976bf2df91509e56d30b54a3bb4f6c
Signed-off-by: Trilok Soni <tsoni@codeaurora.org>