RISC-V: defconfig: Enable printk timestamps

RISC-V: defconfig: Enable printk timestamps

The printk timestamps are very useful information to visually see
where kernel is spending time during boot. It also helps us see
the timing of hotplug events at runtime.

This patch enables printk timestamps in RISC-V defconfig so that
we have it enabled by default (similar to other architectures
such as x86_64, arm64, etc).

Signed-off-by: Anup Patel <anup@brainfault.org>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>