mmc: uniphier-sd: Add control to switch UHS speed

SD interface logic has the register to switch UHS speed.
The default is up to SDR25 and to support SDR50 or faster,
add uniphier_sd_speed_switch() function to switch the speed mode.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Link: https://lore.kernel.org/r/20230125010201.28246-4-hayashi.kunihiko@socionext.com


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