drivers: gpio: xlnx: Add interrupt support

Added support for interrupt-driven GPIO for this driver if an interrupt
line is defined. Note that interrupt support is optional in the GPIO
core so it may or may not be present.

The core provides a single state change notification whenever any line
changes state on a given bank, so we need to check the states of the
lines ourselves to determine what has actually changed.

Also, the driver treats the first and second GPIO banks as mostly
separate devices, but a single interrupt is shared by both banks, so the
ISR ends up being registered by the first bank which also triggers
handling on the second bank if required.

Signed-off-by: Robert Hancock <robert.hancock@calian.com>