mach-ux500: no MMC_CAP_SD_HIGHSPEED on Snowball

MMC_CAP_SD_HIGHSPEED is not supported on Snowball board resulting on
initialization errors.

Cc: stable@kernel.org
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Fredrik Soderstedt <fredrik.soderstedt@stericsson.com>
Signed-off-by: Philippe Langlais <philippe.langlais@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>