Add support for PCA953x I2C gpio devices

Initial support for NXP's 4 and 8 bit I2C gpio expanders
(eg pca9537, pca9557, etc). The CONFIG_PCA953X define
enables support for the devices while the CONFIG_CMD_PCA953X
define enables the pca953x command. The CONFIG_CMD_PCA953X_INFO
define enables an 'info' sub-command which provides summary
information for the given pca953x device.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>