mmc: sdhci: Enable high-speed support for MMC cards

MMC_CAP_MMC_HIGHSPEED allows MMC and eMMC to negotiate up to 50M
instead of the previous limit of 25M.

Signed-off-by: Zhangfei Gao <zgao6@marvell.com>
Acked-by: Matt Fleming <matt@console-pimps.org>
Acked-by: Kyungmin Park <kmpark@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Chris Ball <cjb@laptop.org>