gpio/cs5535: Show explicit dependency between gpio_cs5535 and mfd_cs5535

cs5535-gpio.c has been split into two, with various setup moved
into cs5535-mfd.c.  Given that cs5535-gpio will not load without
the -mfd part, lets make that dependency explicit in Kconfig

Signed-off-by: Ed Wildgoose <kernel@wildgooses.com>
Acked-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>