drivers: gpio: Add PCA953X driver (gpio port expander)

The Texas Instruments TCA9538 is an 8 pin GPIO port expander.
It operates on an I2C bus with 2 configurable address pins. The
device has an interrupt output pin that is asserted when any pin
configured as an input changes state.

Added under the PCA953X name to allow other similar parts to
use the same driver.

Signed-off-by: Kieran Mackey <kieran.mackey@lairdconnect.com>