irqchip/gic-v3: Only provision redistributors that are enabled in ACPI

We currently allocate redistributor region structures for
individual redistributors when ACPI doesn't present us with
compact MMIO regions covering multiple redistributors.

It turns out that we allocate these structures even when
the redistributor is flagged as disabled by ACPI. It works
fine until someone actually tries to tarse one of these
structures, and access the corresponding MMIO region.

Instead, track the number of enabled redistributors, and
only allocate what is required. This makes sure that there
is no invalid data to misuse.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reported-by: Heyi Guo <guoheyi@huawei.com>
Tested-by: Heyi Guo <guoheyi@huawei.com>
Link: https://lore.kernel.org/r/20191216062745.63397-1-guoheyi@huawei.com