irqchip/qcom-pdc: Kill PDC_NO_PARENT_IRQ

PDC_NO_PARENT_IRQ is pretty pointless, as all it indicates is
that the PDC terminates the interrupt hierarchy. Which is
exactly the same as not having a mapping in the GIC space.
This is also bad practice to treat the absence of a hwirq
as a hwirq itself.

Just explicitly use the region mapping pointer, and drop
the definition.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Maulik Shah <quic_mkshah@quicinc.com>
Link: https://lore.kernel.org/r/20220224101226.88373-2-maz@kernel.org