All X-Powers PMICs described by this binding have an IRQ pin, and so
far (almost) all boards connected this to some NMI pin or GPIO on the SoC
they are connected to.
However we start to see boards that omit this connection, and technically
the IRQ pin is not essential to the basic PMIC operation.

The existing Linux driver allows skipping the IRQ pin setup for two chips
already, so update the binding to also make the DT property optional for
the missing chip. And while we are at it, add the AXP313a to that list,
as they are actually boards out there not connecting the IRQ pin.

This allows to have DTs correctly describing those boards not wiring up
the interrupt.
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230919103913.463156-2-andre.przywara@arm.comSigned-off-by: Lee Jones <lee@kernel.org>