OF: add MMC binding helpers

Hi all,

in working to add OF support to S3C MMC I discovered that some good
code has been already written for the MMC-over-SPI driver. This is the first attempt to use it to build a common set of OF bindings for MMC drivers.

Please tell me whether I'm on a good way or not.

cheers,
Domenico

-----[ Domenico Andreoli, aka cavok
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