PCI: Document hpiosize= and hpmemsize= resource reservation parameters

Document PCI hotplug resource reservation parameters hpiosize and
hpmemsize.  These parameters override default hotplug I/O size (256 bytes)
and default mem size (2M).
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

Documentation/kernel-parameters.txt

@@ -2248,6 +2248,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 				the default.
 				off: Turn ECRC off
 				on: Turn ECRC on.
+		hpiosize=nn[KMG]	The fixed amount of bus space which is
+				reserved for hotplug bridge's IO window.
+				Default size is 256 bytes.
+		hpmemsize=nn[KMG]	The fixed amount of bus space which is
+				reserved for hotplug bridge's memory window.
+				Default size is 2 megabytes.
 		realloc=	Enable/disable reallocating PCI bridge resources
 				if allocations done by BIOS are too small to
 				accommodate resources required by all child