platform-msi: Add ABI to show msi_irqs of platform devices

PCI devices expose the associated MSI interrupts via sysfs, but platform
devices which utilize MSI interrupts do not. This information is important
for user space tools to optimize affinity settings.

Utilize the generic MSI sysfs facility to expose this information for
platform MSI.

Signed-off-by: Barry Song <song.bao.hua@hisilicon.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210813035628.6844-3-21cnbao@gmail.com