ARM: dts: aspeed: Add LPC reset controller node

On both the ast2400 and ast2500 SoCs, the LPC reset controller is
required to bring the UARTs out of reset without waiting for the LPC
reset to be deasserted.

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