dt-bindings: mfd: adp5585: document adp5589 I/O expander

dt-bindings: mfd: adp5585: document adp5589 I/O expander

The ADP5589 is a 19 I/O port expander with built-in keypad matrix decoder,
programmable logic, reset generator, and PWM generator.

We can't really have adp5589 devices fallback to adp5585 (which have
less pins) because there are some significant differences in the register
map.

Reviewed-by: "Rob Herring (Arm)" <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20250701-dev-adp5589-fw-v7-5-b1fcfe9e9826@analog.com
Signed-off-by: Lee Jones <lee@kernel.org>