irqchip/gic-v3: Ignore disabled ITS nodes

[ Upstream commit 95a2562590c2f64a0398183f978d5cf3db6d0284 ]

On some platforms there's an ITS available but it's not enabled
because reading or writing the registers is denied by the
firmware. In fact, reading or writing them will cause the system
to reset. We could remove the node from DT in such a case, but
it's better to skip nodes that are marked as "disabled" in DT so
that we can describe the hardware that exists and use the status
property to indicate how the firmware has configured things.

Cc: Stuart Yoder <stuyoder@gmail.com>
Cc: Laurentiu Tudor <laurentiu.tudor@nxp.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Rajendra Nayak <rnayak@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>