s390: Avoid call to irq_reserve_irqs()

There is no need to mark the lower interrupts as reserved in order to
exclude them from dynamic allocation.

Provide arch_dynirq_lower_bound() to exclude the lower space.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Grant Likely <grant.likely@linaro.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: linux390@de.ibm.com
Link: http://lkml.kernel.org/r/20140507154339.811205235@linutronix.de


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>