serial: core: Add support of runtime PM

8250 driver has wrong implementation of runtime power management, i.e.
it uses an irq_safe flag. The irq_safe flag takes a permanent usage count
on the parent device preventing the parent from idling. This patch
prepares for making runtime power management generic by adding runtime PM
calls to serial core once for all UART drivers.

As we have serial drivers that do not enable runtime PM, and drivers that
enable runtime PM, we add new functions for serial_pm_resume_and_get() and
serial_pm_autosuspend() functions to handle errors and allow the use also
for cases when runtime PM is not enabled. The other option considered was
to not check for runtime PM enable errors. But some CPUs can hang when the
clocks are not enabled for the device, so ignoring the errors is not a good
option. Eventually with the serial port drivers updated, we should be able
to just switch to using the standard runtime PM calls with no need for the
wrapper functions.

Note that this patch only adds runtime PM calls to the functions where we
can call them for synchronous wake-up without the need for irq_safe flag.
Additionally we also need asynchronous wake-up support for __uart_start(),
that is added in a separate patch.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Co-developed-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>