ACPI: NVS: fix doc warnings in nvs.c

Fixes the following W=1 kernel build warning(s):

 drivers/acpi/nvs.c:94: warning: Function parameter or
  member 'start' not described in 'suspend_nvs_register'
 drivers/acpi/nvs.c:94: warning: Function parameter or
  member 'size' not described in 'suspend_nvs_register'

Signed-off-by: Baokun Li <libaokun1@huawei.com>
[ rjw: Subject edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>