PNP: system.c: unmark a comment as being kernel-doc

Fix a documentation build warning caused by the comment not being
in kernel-doc format:

system.c:110: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Reserve motherboard resources after PCI claim BARs,

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>