ARM: dts: aspeed: mtmitchell: Enable the BMC UART8 and UART9

The BMC UART8 and UART9 were connected to the Secpro and Mpro console
of socket S1 on the Mt.Mitchell system.

Signed-off-by: Chanh Nguyen <chanh@os.amperecomputing.com>
Link: https://lore.kernel.org/r/20230620092537.20007-2-chanh@os.amperecomputing.com


Signed-off-by: Joel Stanley <joel@jms.id.au>