ARM: dts: aspeed: Add device tree for Ampere's Mt. Mitchell BMC

The Mt. Mitchell BMC is an ASPEED AST2600-based BMC for the Mt. Mitchell
hardware reference platform with AmpereOne(TM) processor.
Signed-off-by: Quan Nguyen <quan@os.amperecomputing.com>
Signed-off-by: Phong Vo <phong@os.amperecomputing.com>
Signed-off-by: Thang Q. Nguyen <thang@os.amperecomputing.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20220817071539.176110-3-quan@os.amperecomputing.comSigned-off-by: Joel Stanley <joel@jms.id.au>

arch/arm/boot/dts/Makefile

@@ -1576,6 +1576,7 @@ dtb-$(CONFIG_ARCH_ASPEED) += \
 	aspeed-ast2600-evb.dtb \
 	aspeed-bmc-amd-ethanolx.dtb \
 	aspeed-bmc-ampere-mtjade.dtb \
+	aspeed-bmc-ampere-mtmitchell.dtb \
 	aspeed-bmc-arm-stardragon4800-rep2.dtb \
 	aspeed-bmc-asrock-e3c246d4i.dtb \
 	aspeed-bmc-asrock-romed8hm3.dtb \