arm64: defconfig: enable CONFIG_RANDOMIZE_BASE

For testing coverage and improved defense in depth, enable KASLR by
default.

Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Will Deacon <will@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>