arm64: defconfig: Enable CONFIG_PREEMPT by default

This way we can spot early bugs when just testing with the default
config.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: dab7ea360967f26337be1f26f56b12771da8c731
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Ian Maund <imaund@codeaurora.org>