drivers: gpio: pca95xx: add latch for interrupt pins

Basically, once a pin is configured for interrupt, we enable the latch
on it, in order not to miss short pulses.
The bottom half of the interrupt then reads the input port twice, to
read the latch and then read the potentially new state of the pins, to
compute the posedge, negedge, and level callbacks accordingly.

Signed-off-by: Vincent Geneves <vgeneves@kalray.eu>