ACPI,PCI,IRQ: reduce static IRQ array size to 16

Now that the supported number of PCI IRQs are no longer capped
with 256, renaming the static array to support ISA IRQs only
and removing the MAX_IRQS constant.

Signed-off-by: Sinan Kaya <okaya@codeaurora.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>