ARM: dts: aspeed: Clean up UART nodes

- Shorten size of reg property so it covers only the implemented
 registers

 - Add VUART compatible, and change node name to serial@

 - Remove outdated current-speed property. Different bootloaders use
 different speeds, so this is no longer helpful

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