ARM: dts: aspeed: Add aliases for UARTs

Existing userspace expects the console (UART5) to be at /dev/ttyS4.  To
ensure the UARTs show up where users expect them, we give them fixed
aliases starting at 0.

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