gpio: pca953x: Add support for PCA9556

PCA9556 is the software-compatible predecessor to the PCA9557, so add it
to the supported I2C device ID table.

Signed-off-by: Nate Case <ncase@xes-inc.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>