MMC: Consolidate MMC/SD command definitions

This moves the MMC and SD Card command definitions from
include/asm/arch/mmc.h into include/mmc.h. These definitions are
given by the MMC and SD Card standards, not by any particular
architecture.

There's a lot more room for consolidation in the MMC drivers which
I'm hoping to get done eventually, but this patch is a start.

Compile-tested for all avr32 boards as well as lpc2292sodimm and
lubbock. This should cover all three mmc drivers in the tree.

Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>