gpio: timberdale: simplify dependencies

GPIO_TIMBERDALE doesn't need an explicit dependency on HAS_IOMEM,
because it depends on MFD_TIMBERDALE which itself depends on
HAS_IOMEM already.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Alexandre Courbot <gnurou@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>