mmc: jz4740: Remove platform data and use standard APIs

Drop the custom code to get the 'cd' and 'wp' GPIOs. The driver now
calls mmc_of_parse() which will init these from devicetree or
device properties.

Also drop the custom code to get the 'power' GPIO. The MMC core
provides us with the means to power the MMC card through an external
regulator.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>