defconfig: sm8150: Enable ICE metadata encryption

Enable the device-mapper target "dm-default-key" which assigns
an encryption key to bios that don't already have one. This will
be used on Android's userdata partition, so that all data not
already encrypted with ext4 encryption is still encrypted with a
default key.

Change-Id: If6ef872019b1ae489ff1122a56ced0917a9aae85
Signed-off-by: Shivaprasad Hongal <shongal@codeaurora.org>