python: Add JsonLoader to pystats

Allows for loading JSON files (e.g. m5out/stats.json) into the gem5
Python stats model.

Change-Id: I948dff0b23ea65c4c7acaac5e4fdf3795de1c0a7
Signed-off-by: Kelly Nguyen <klynguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/43606
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/python/m5/ext/pystats/__init__.py

@@ -30,6 +30,7 @@ from .simstat import SimStat
 from .statistic import Statistic
 from .storagetype import StorageType
 from .timeconversion import TimeConversion
+from .jsonloader import JsonLoader
 
 __all__ = [
            "Group",
@@ -38,4 +39,5 @@ __all__ = [
            "TimeConversion",
            "StorageType",
            "JsonSerializable",
-          ]
+           "JsonLoader",
+          ]