mmc: Split mmc struct, rework mmc initialization (v2)

The way that struct mmc was implemented was a bit of a mess;
configuration and internal state all jumbled up in a single structure.

On top of that the way initialization is done with mmc_register leads
to a lot of duplicated code in drivers.

Typically the initialization got something like this in every driver.

	struct mmc *mmc = malloc(sizeof(struct mmc));
	memset(mmc, 0, sizeof(struct mmc);
	/* fill in fields of mmc struct */
	/* store private data pointer */
	mmc_register(mmc);

By using the new mmc_create call one just passes an mmc config struct
and an optional private data pointer like this:

	struct mmc = mmc_create(&cfg, priv);

All in tree drivers have been updated to the new form, and expect
mmc_register to go away before long.

Changes since v1:

* Use calloc instead of manually calling memset.
* Mark mmc_register as deprecated.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>