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 mtmitch

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

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