ACPI: docs: enumeration: Update UART serial bus resource documentation

In some cases UART serial bus resource may be represented by struct
serdev_device.

Fixes: 53c76263

 ("serdev: Add ACPI support")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>