arm: GIC initialisation skeleton

arm: GIC initialisation skeleton

Since the GIC uses MMIOs, its initialisation must be done at EL2. This
is why arch_cpu_init first calls irqchip_init on the master CPU, to map
the devices, and then irqchip_cpu_init on all CPUs.

The aim of this patch is to allow support for both GICv2 and GICv3. It
abstracts the GIC operations by using `struct irqchip_ops', and fills it
with the right device hooks after detecting which irqchip is available.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>