regulator: ltc3589: make IRQ optional

regulator: ltc3589: make IRQ optional

On the AM335x SoC rev. <= 1.0 the "Input Function of the EXTINTn
Terminal is Inverted", for which the only remedy is to "Use an active
high interrupt source or use an external inverter to change the
polarity of any active low interrupt source."

This pin is used as IRQ pin for the LTC3589 PMIC on the Ka-Ro
electronics TX48 module. Make the IRQ optional in the driver and use a
polling routine instead if no IRQ is specified in DT.
Otherwise the driver will continuously generate interrupts and make
the system unusable.