mmc: sdhci-pci-gli: Add a switch to enable/disable SSC for GL9750 and GL9755

Add a vendor-specific bit at the bit26 of GL9750's register 878h and
GL9755's register 78h to decide whether to disable SSC function. If
this bit is set, the SSC function will be disabled.

Signed-off-by: Ben Chuang <ben.chuang@genesyslogic.com.tw>
Link: https://lore.kernel.org/r/20220119075406.36321-1-benchuanggli@gmail.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>