mmc: starfive: Change tuning implementation

Before, we used syscon to achieve tuning, but the actual measurement
showed little effect, so the tuning implementation was modified here,
and it was realized by reading and writing the UHS_REG_EXT register.

Signed-off-by: William Qiu <william.qiu@starfivetech.com>
Reviewed-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Link: https://lore.kernel.org/r/20230922062834.39212-3-william.qiu@starfivetech.com


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