mmc: Change mode when switching to a boot partition

Boot partitions do not support HS200. Changing to a lower performance mode
is required to access them.
mmc_select_mode_and_width() and sd_select_mode_and_width() are modified to
make it easier to call them outside of the initialization context.

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