x86/acpi: call mp_config_acpi_gsi() in mp_register_gsi()

The patch to call mp_config_acpi_gsi() from the ACPI IRQ registration
code never got mainline because there were open discussions about it.

This call is needed to properly update the kernel's copy of the mptable,
when the update_mptable boot parameter is needed.

Now that the dust has settled with the APIC unification, and since there
were no objections when the patch was re-submitted, try this again.

[ Impact: fix the update_mptable boot parameter ]

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Cc: Len Brown <lenb@kernel.org>
LKML-Reference: <4A01C387.7090103@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>