gpio: Kconfig: drop unknown symbol 'CS5535_GPIO'

Commit af0c25c4

 ("staging: remove obsoleted CS5535/CS5536 GPIO
driver") dropped CS5535_GPIO. So this last reference to CS5535_GPIO has
been an (inconsequential) nop since v3.1. Drop it too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>