ARM: dts: aspeed: Modify I2C bus configuration

Enable I2C bus 8 which is exposed on the IPMB_1 connector on the X570D4U
mainboard.

Additionally adds a descriptive comment to I2C busses 1 and 5.

Signed-off-by: Renze Nicolai <renze@rnplus.nl>
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>