resources: Document IORESOURCE_IO

Help clarify that this is specifically for PCI/ISA I/O ports and not for
any other similar thing.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Tested-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>