configurations: Add Mt.Mitchell's cards

This commit add Mt.Mitchell's card configurations:
1. PowerSupply board to mtmitchell_mb.json
2. Backplane card to mtmitchell_bp.json
3. Riser card to mtmitchell_riser.json

Test: Latest CI build passed

Signed-off-by: Chau Ly <chaul@amperecomputing.com>
Change-Id: Idd8a06262af0c362cc5f76bb36df63303f75f46e
diff --git a/meson.build b/meson.build
index 555de7a..78ab453 100644
--- a/meson.build
+++ b/meson.build
@@ -154,10 +154,12 @@
     'mori_bmc.json',
     'mori_motherboard.json',
     'mtjade.json',
-    'mtmitchell_bmc.json',
-    'mtmitchell_mb.json',
     'mtjefferson_bmc.json',
     'mtjefferson_mb.json',
+    'mtmitchell_bmc.json',
+    'mtmitchell_bp.json',
+    'mtmitchell_mb.json',
+    'mtmitchell_riser.json',
     'mudflap.json',
     'nf5280m7_baseboard.json',
     'nisqually.json',