x86/madt: make the interrupt override parser a public function

x86/madt: make the interrupt override parser a public function

Split a portion of the code in madt_parse_interrupt_override to a
separate function, that is public and can be used from other code.
This will be needed by the Xen port, since FreeBSD needs to parse the
interrupt overrides and notify Xen about them.

This commit should not introduce any functional change.

Sponsored by: Citrix Systems R&D
Reviewed by: jhb, gibbs

x86/acpica/madt.c:
 - Introduce madt_parse_interrupt_values() that parses the intr
   information from ACPI and returns the triggering and the polarity.
   This is a subset of the functionality that used to be part of
   madt_parse_interrupt_override().
 - Make madt_found_sci_override a global variable that can be used
   from other files.

x86/include/acpica_machdep.h:
 - Prototype of madt_parse_interrupt_values.
 - Extern declaration of madt_found_sci_override.