overlays: Fix mcp23017's addr parameter

The addr parameter of the mcp23017 overlay was broken by the addition
of the noints parameter; splitting the mcp node in two without also
modifying the second half from the addr parameter would cause the two
halves to separate. Change the implementation strategy to patch
fragment 2 (as was originally proposed). This will prevent the
overlay from being applied at runtime until the "dtoverlay" command
is improved, but the overlay already has this restriction due to
fragment 3 so this isn't a step backwards.

See: https://github.com/raspberrypi/linux/issues/3449



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