dt-bindings: aspeed-lpc: Add AST2600 compatible strings

The AST2600 datasheet indicates the same register set behind the LPC
controller address space.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
Reviewed-by: Andrew Jeffery <andrew@aj.id.au>
Link: https://lore.kernel.org/r/20190925125610.12096-2-bradleyb@fuzziesquirrel.com


Signed-off-by: Joel Stanley <joel@jms.id.au>