ACPI / PCI: Do not call ISA-specific code if ISA is not supported

This commit moves ISA-specific code to separate function and makes that
function depend on CONFIG_{E}ISA so that we do not have to maintain
acpi_isa_irq_to_gsi() function for architectures which do not support ISA.

Signed-off-by: Tomasz Nowicki <tomasz.nowicki@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>