dm: serial: Add ->getconfig() callback

In some cases it would be good to know the settings, such as parity,
of current serial console. One example might be an ACPI SPCR table
to generate using these parameters.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>