OMAP: mmc: add support for second and third mmc channels

This patch adds support for the second and third mmc channels on OMAP3
processors

Boards wishing to use this feature should define CONFIG_SYS_MMC_SET_DEV
in the board config

Tested on Overo

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Tested-by: Philip Balister <philip@opensdr.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>