dev: Use lambdas instead of the Callback type for serial devices.

dev: Use lambdas instead of the Callback type for serial devices.

Issue-on: https://gem5.atlassian.net/browse/GEM5-698
Change-Id: Idb87fa0b90d14981fd61f997285f61b2ef304227
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32647
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>