dev: Replace the Callback class with lambdas in ARM's flash devices.

dev: Replace the Callback class with lambdas in ARM's flash devices.

Issue-on: https://gem5.atlassian.net/browse/GEM5-698
Change-Id: I2694dd1952b7412c27c83c9d15d4645899bd28e2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/32648
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>