mmc: refactor MMC startup to make it easier to support new modes

The MMC startup process currently handles 4 modes. To make it easier to
add support for more modes, let's make the process more generic and use a
list of the modes to try.
The major functional change is that when a mode fails we try the next one.
Not all modes are tried, only those supported by the card and the host.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>