dt-bindings: interrupt-controller: arm,gic-v3: Describe ESPI range support

GICv3.1 introduces support for new interrupt ranges, one of them being
the Extended SPI range (ESPI). The DT binding is extended to deal with
it as a new interrupt class.

Reviewed-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>