serial: lantiq: Add support for Lightning Mountain SoC

This patch adds IRQ & ISR support in the driver for Lightning Mountain SoC.

Signed-off-by: Rahul Tanwar <rahul.tanwar@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Link: https://lore.kernel.org/r/0df20f6e4bbf9de09c85a5c92c92e642f62f441f.1565257887.git.rahul.tanwar@linux.intel.com


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