mmc: slot-gpio: Make mmc_gpio_alloc() available for MMC core

As a step in moving slot-gpio functions/structs closer to the MMC core,
let's add a local header file for slot-gpio.

In this initial step we move mmc_gpio_alloc() into the header file,
to make it available for the MMC core. Following patches will show the
utilization of it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>