defconfig: msm: use codename for sdm640 defconfigs

use code name for sdm640 defconfig files so that
android builds doesn't have to expose soc name.

Change-Id: I603708d9a49439f7899b6bce36741c79dfd4df95
Signed-off-by: Srinivas Ramana <sramana@codeaurora.org>