ACPI: APD: Add kerneldoc for properties in struct apd_device_desc

commit 7ff55d17 ("ACPI / APD: Provide build-in properties of the
UART") added the struct property_entry *properties field to struct
apd_device_desc, but didn't add the kernel doc for it, so add it now.

Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
[ rjw: Subject and changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>