OMAP: Clean up MMC driver, part 1

- Move register definitions away from the header file
- Change OMAP_MMC_{READ,WRITE} macros to use the host pointer
- Replace base with virt_base and phys_base
- Implement OMAP2420 initialization workaround
- General code cleanups

Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>