mmc: sdhci: Write to HOST_CONTROL2 register for HS400 speed mode

Enable HS400 speed mode by writing to HOST_CONTROL2 register.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Link: https://lore.kernel.org/r/20210405144428.12159-1-a-govindraju@ti.com