ARM: configs: multi_v7: Add ASPEED G6

This adds the ASPEED AST2600 system and associated ASPEED devices so we
get build coverage.

The changes to the UART configuration to ensure the default console
(UART5) works.

Acked-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>