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

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

The SDcard startup process currently handles only 2 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>