arm: irqchip: add SPI configuration in cell_init and cell_exit

arm: irqchip: add SPI configuration in cell_init and cell_exit

This patch enables the routing of SPIs to the new cell's first CPU. When
destroyed, all SPIs are re-routed to the root cell.
An exhaustive implementation would save the targets of each IRQ before
transferring it to a new cell. Since linux does not currently route SPIs
to secondary CPUs and the root cell is not supposed to use devices that
will be assigned to guests anyway, it should be safe to route everything
to CPU0.

This patch follows the core configuration and the IOAPIC implementation,
which only allows to use the first 64 SPIs.
A future patch will need to change this minimal bitmap size to 988.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
[Jan: switch to mmio accessors]
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>