Convert acpiphp to be builtin only, with no module option.

Previously, when HOTPLUG_PCI_ACPI=m, users could disable acpiphp by
removing the module or preventing it from loading.  That can't be done
if acpiphp is builtin statically, so this adds an "acpiphp.disable"
kernel parameter.  If a user needs to use this parameter, it is a bug,
and we want to hear about it.

[bhelgaas: fold in acpiphp.disable here, remove documentation]
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>