defconfig: Enable default-key driver for msm8937_64

This flag is needed for Metadata encryption to work:
CONFIG_DM_DEFAULT_KEY=y.
Test: 1. Device bootup with Metadata Encryption Enabled.
      2. Randomness test.
      3. vts_kernel_encryption_tests

Change-Id: I42d9ea2b93e5731e4047383e8ad120ebe311200a
Signed-off-by: Vaibhav Agrawal <vagrawa@codeaurora.org>