omap mmc: Add better MMC low-level init

This will simplify the MMC low-level init, and make it more
flexible to add support for a newer MMC controller in the
following patches.

The patch rearranges platform data and gets rid of slot vs
controller confusion in the old data structures. Also fix
device id numbering in the clock code.

Some code snippets are based on an earlier patch by
Russell King <linux@arm.linux.org.uk>.

Cc: Pierre Ossman <drzeus-mmc@drzeus.cx>
Signed-off-by: Tony Lindgren <tony@atomide.com>