drivers: mmc: iproc_sdhci: enable HS200 mode

Add tuning functionality which is needed for HS200 mode.
For HS200, program the correct needed 1.8 voltage

Signed-off-by: Bharat Gooty <bharat.gooty@broadcom.com>
Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>