ARM: dts: aspeed: Add Everest BMC machine

Add the Everest machine powered by an AST2600 chip. Copy the basic
configuration from Rainier.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Tested-by: Andrew Geissler <geissonator@gmail.com>
Link: https://lore.kernel.org/r/20201211170704.27640-1-eajames@linux.ibm.com


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