pseries: added allocator for a block of IRQs

The patch adds a simple helper which allocates a consecutive sequence
of IRQs calling spapr_allocate_irq for each and checks that allocated
IRQs go consequently.

The patch is required for upcoming support of MSI/MSIX on POWER.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
