mmc: cavium: Depend on GPIO driver

Without the ThunderX/OcteonTx GPIO driver the MMC driver
would not power up any MMC devices. Therefore add a
dependency to the GPIO driver and remove the unneeded GPIOLIB
dependency.

Signed-off-by: Jan Glauber <jglauber@cavium.com>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>