gpio: pca953x driver handles pca9554 too

Teach drivers/gpio/pca953x.c about PCA9554, another compatible chip.

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>