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