gpio/pca953x: Deprecate meaningless device-tree bindings

The property 'polarity' is handled by the GPIO core, and the 'gpio-base'
should be assigned automatically. It is meaningless in the device-tree,
since GPIO's are identified by the "chip-name"/offset pair.
This way, the whole pca953x_get_alt_pdata() can hopefully soon go away.
We still need to check whether we really want GPIO-interrupt functionality
by simply looking if the I2C node has an interrupts property defined, since
this property is not used for anything else.

Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>