defconfig: snapshot mdm9640 config

Snapshot the mdm9640 defconfig from msm-3.10 as of
0ab0c5080b30dbd1d26948fde7299e066b6edd9b
("msm: ipa: fix rx bytes reporting on IPA WAN driver")

Also rename the defconfig to mdm_defconfig to enable sharing
with other similar mdm chipsets.

Change-Id: Id4967c27795fd3b734f2739d535e64bcab0b6484
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>