drivers: gpio: sn74hc595: fix terminology for GPIO expander

The Kconfig help text for the SN74HC595 driver refers to the device
as a "GPIO extender", which is inconsistent with the devicetree
binding file (ti,sn74hc595.yaml) that describes it as a "GPIO expander".

Signed-off-by: Hank Wang <wanghanchi2000@gmail.com>