mmc: tegra: fix eMMC DDR52 mode

Make sure the clock is doubled when using eMMC DDR52 mode.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Tested-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>