mmc: uniphier-sd: Add control of UHS mode using SD interface logic

Transition of UHS mode needs to control the register in SD interface
logic. Add access to the register in the logic using the regmap from
"socionext,syscon-uhs-mode" property.

Define the start_signal_voltage_switch function only if UHS mode is
available.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/20230125010201.28246-3-hayashi.kunihiko@socionext.com


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