s390/irq: improve displayed interrupt order in /proc/interrupts

Rework the irqclass_main_desc and irqclass_sub_desc data structures which
are used to report detaild IRQ statistics in /proc/interrupts.  When
called from the procfs ops, the entries in the structures are processed
one by one.

The index of an IRQ in the structures is identical to its definition in
the "enum interruption_class".   To control and (re)order the displayed
sequence, introduce an irq member in each entry.  This helps to display
related IRQs together without changing the assigned number in the
interruption_class enumeration.  That means, adding and displaying new
IRQs are independent.
Finally, this new behavior improves to maintain a kernel ABI.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>