defconfig: Enable blk-crypto-fallback to handle sw crypto request

For data encryption where inline crypto engine is not available,
like external storage slots, blk-crypto should fallback to
software crypto to handle such requests.

Change-Id: I5dcfd424574fb0d5e773979bf422c6a39de3fe9c
Signed-off-by: Phanindra Babu Pabba <pabba@codeaurora.org>