misc: Update MAINTAINERS with learning-gem5 tag

Change-Id: Ic91fb1d9b2c3c42946cb84c1ec52d9376d4e50b4
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/6422
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>

MAINTAINERS

@@ -58,6 +58,10 @@ ext: Components external to gem5
 gpu-compute:
   Tony Gutierrez <anthony.gutierrez@amd.com>
 
+learning-gem5: The code and configs for the Learning gem5 book (see
+               learning.gem5.com)
+  Jason Lowe-Power <jason@lowepower.com>
+
 mem: General memory system (e.g., XBar, Packet)
   Nikos Nikoleris <nikos.nikoleris@arm.com>
 mem-cache: Classic caches and coherence