gpio: handle pca953{4,5,6,7,8} too

This third part of an extension to support more pca953x chips updates the
logic to handle the smaller register widths used by the 4-bit and 8-bit parts,
and to use the chip type to determine how many GPIOs it provides.

As long as we don't support interrupt and reset capabilities, those size
issues are the only software-visible differences between these parts.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@pengutronix.de>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>