fixup! overlays: mcp23017: allow specification of the i2c bus

The incorrect fragment order (*) caused broke the interrupt usage, and
while it was being fixed the lack of a reference to the pinctrl
declaration was noticed.

See: https://github.com/raspberrypi/linux/issues/5677



Signed-off-by: Phil Elwell <phil@raspberrypi.com>

(*) Ideally all fragments would appear in the file in the order in which
they should be merged, but that is easy to forget and can be awkward, so
the firmware merges all "intra" fragments (those that target other
fragments in the overlay) before "inter" fragments (those that target
the base DTB). However, intra fragments that target other intra
fragments is a level of nesting too far for this logic to cope, so they
must appear before the fragments they target.