gpio: pca953x: add support for pca9555 I2C I/O expander

Add support for pca9555 I2C I/O expander.  As the comment suggests this part
is software compatible with the pca9539.

Signed-off-by: Will Newton <will.newton@imgtec.com>
Cc: "eric miao" <eric.miao@marvell.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>