Add support for the GL9750 and GL9755 chipsets.

Enable v4 mode and wait 5ms after set 1.8V signal enable for GL9750/
GL9755. Fix the value of SDHCI_MAX_CURRENT register and use the vendor
tuning flow for GL9750.

Co-developed-by: Michael K Johnson <johnsonm@danlj.org>
Signed-off-by: Michael K Johnson <johnsonm@danlj.org>
Signed-off-by: Ben Chuang <ben.chuang@genesyslogic.com.tw>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>