dev: Add a new RegisterBank which helps handle device registers.

This change includes both the RegisterBank class and register classes,
and a unit test which exercises them.

Change-Id: I28ef0c0b9192ad786625ac83f096f69d8e5af00f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/35856
Reviewed-by: Matthew Poremba <matthew.poremba@amd.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/dev/SConscript

@@ -54,3 +54,5 @@ Source('pixelpump.cc')
 
 DebugFlag('Intel8254Timer')
 DebugFlag('MC146818')
+
+GTest('reg_bank.test', 'reg_bank.test.cc')