arm64: defconfig: Enable printk timestamps

Timestamps in the kernel log can be useful to quickly determine where
time is spent during boot. There are tools that can generate charts of
the boot time, but it's sometimes convenient to see this live and
without the need to go through scripts to parse the logs first.

Also, this option is enabled on a large number of configurations, not
enabling it is confusing because the boot log looks very different from
what one is used to.

Signed-off-by: Thierry Reding <treding@nvidia.com>