AHCI: Support multiple MSIs

Take advantage of multiple MSIs implementation on x86 - on
systems with IRQ remapping AHCI ports not only get assigned
separate MSI vectors - but also separate IRQs. As result,
interrupts generated by different ports could be serviced on
different CPUs rather than on a single one.

In cases when number of allocated MSIs is less than requested
the Sharing Last MSI mode does not get used, no matter
implemented in hardware or not. Instead, the driver assumes the
advantage of multiple MSIs is negated and falls back to the
single MSI mode as if MRSM bit was set (some Intel chips
implement this strategy anyway - MRSM bit gets set even if the
number of allocated MSIs exceeds the number of implemented ports).

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Acked-by: Jeff Garzik <jgarzik@redhat.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Matthew Wilcox <willy@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/15bf7ee314dd55f21ec7d2a01c47613cd8190a7c.1353324359.git.agordeev@redhat.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>