pca953x: support 16-pin devices

This adds support for for the PCA9535/PCA9539 family of gpio devices which
have 16 output pins.

To let the driver know which devices are 16-pin it is necessary to define
CONFIG_SYS_I2C_PCA953X_WIDTH in your board config file. This is used to
create an array of {chip, ngpio} tuples that are used to determine the
width of a particular chip. For backwards compatibility it is assumed that
any chip not defined in CONFIG_SYS_I2C_PCA953X_WIDTH has 8 pins.

Acked-by: Peter Tyser <ptyser@xes-inc.com>
Tested-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>