PCI: Allow additional bus numbers for hotplug bridges

A user may hot add a switch requiring more than one bus to enumerate.  This
previously required a system reboot if BIOS did not sufficiently pad the
bus resource, which they frequently don't do.

Add a kernel parameter so a user can specify the minimum number of bus
numbers to reserve for a hotplug bridge's subordinate buses so rebooting
won't be necessary.

The default is 1, which is equivalent to previous behavior.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

Documentation/kernel-parameters.txt

@@ -3016,6 +3016,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 		hpmemsize=nn[KMG]	The fixed amount of bus space which is
 				reserved for hotplug bridge's memory window.
 				Default size is 2 megabytes.
+		hpbussize=nn	The minimum amount of additional bus numbers
+				reserved for buses below a hotplug bridge.
+				Default is 1.
 		realloc=	Enable/disable reallocating PCI bridge resources
 				if allocations done by BIOS are too small to
 				accommodate resources required by all child