serial: core: Introduce uart_console_enabled() helper

Introduce uart_console_enabled() helper which checks port to be console
and console is registered in the list.

Note, this helper will be used in the future as well.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200214114339.53897-2-andriy.shevchenko@linux.intel.com


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