regulator: pf9453: Fix IRQ trigger and allow

Merge series from Franz Schnyder <franz.schnyder@toradex.com>:

The IRQ_B pin is an open-drain output. The datasheet specifies, that the
IRQ_B pin is pulled low when any unmasked interrupt bit status
is changed, and it is released high once the application processor reads
the INT1 register. As it specifies a level-low behavior, it should not
force a falling-edge interrupt.

Remove the IRQF_TRIGGER_FALLING to not force the falling-edge interrupt
and instead rely on the flag from the device tree.
Set the IRQF_SHARED to be able to share the interrupt line with
other devices. If the interrupt has not been triggered by the PMIC,
return IRQ_NONE.