base: Add static assert to trie

base: Add static assert to trie

Trie is based on the condition that they key is an integral. As so,
add a static_assert to guarantee that. Also, added some general info
about tries to the class.

Change-Id: Idc18f6a685db8047cd44b791f427666d3dd2d187
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26784
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>