mmc: mmci: augment driver to handle gpio descriptors

Currently the MMCI driver will only handle GPIO descriptors
implicitly through the device tree probe glue in mmc_of_init(),
but devices instatiated other ways such as through board files
and passing descriptors using the GPIO descriptor table will
not be able to exploit descriptors.

Augment the driver to look for a GPIO descriptor if device
tree is not used for the device, and if that doesn't work,
fall back to platform data GPIO assignment using the old
API. The end goal is to get rid of the platform data integer
GPIO assingments from the kernel.

This enable the MMCI-embedding platforms to be converted to
GPIO descritor tables.

Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>