i2c: Aspeed: Add AST2600 compatible

The driver default behavior works with the AST2600. We need a new
compatible though to make sure the driver doesn't enable AST2400 or
AST2500 behavior.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>