mmc: dw_mmc: exynos: Warn if HS400 is being used on non-Exynos5420 chipset

Chipsets before Exynos5420 did not support HS400 so if MMC core tries to
configure HS400 timing, this might or might not work.  Warn in such
cases because this is DTB misconfiguration.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>