I/O Expander: Remove hard-coded PCA9555 fields from ioexpander.h definitons.  Add support for an attach() method that may be used when any subset of pin interrupts occur.

PCA9555 Driver:  Replace the signalling logic with a simple callback using the new definitons of ioexpander.h.  This repartitioning of functionality is necessary because (1) the I/O expander driver is the lower-lower part of any driver that uses GPIOs (include the GPIO driver itself) and should not be interacting directly with the much higher level application layer.  And (2) in order to be compatible with the GPIO driver (and any arbitrary upper half driver), the PCA9555 should not directly signal, but should call back into the upper half.  The upper half driver that interacts directly with the application is the appropriate place to be generating signal.