serial: lantiq: Use proper DT compatible string

Use explicit string instead of a macro for devicetree compatible string.

This series of patches is to add support for multiple SoCs which reuse the same
serial controller IP. The following patches will add another compatible string
to support new Lightning Mountain(LGM) SoC. So it makes sense to have the
compatible strings explicitly mentioned instead of a fixed macro.

Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Rahul Tanwar <rahul.tanwar@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Link: https://lore.kernel.org/r/57e2b69e9fbd93328a477b4c7dd2dcc78784ecb1.1565257887.git.rahul.tanwar@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>