eeprom/at2x: add config to disable handling WP

There is a part of the AT2X driver that handles controlling WP pin
connected to the EEPROM chips, but in some systems, the WP line can be
controlled by another component.

Check if any AT2X node defines wp-gpios, if not - do not compile the
code related to WP to save space.

Signed-off-by: Dawid Niedzwiecki <dn@semihalf.com>