dev: Move generic serial devices to src/dev/serial

Change-Id: I104227fc460f8b561e7375b329a541c1fce881b2
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/4291
Reviewed-by: Gabe Black <gabeblack@google.com>