serial: lantiq: Add SMP support

The existing driver can only support single core SoC. But new multicore
platforms which reuse the same driver/IP need SMP support. This patch adds
multicore support in the driver.

Signed-off-by: Rahul Tanwar <rahul.tanwar@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Link: https://lore.kernel.org/r/7912786cccad60c72b20ea724af1def505ab22aa.1565160764.git.rahul.tanwar@linux.intel.com


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