mmc: core: Indicate SD specs higher than 4.0

SD specs version 4.x and 5.x have a dedicated slices in the SCR register.
Higher versions will rely on a combination of the existing fields.

Signed-off-by: Avri Altman <avri.altman@wdc.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>