mmc: sdhci: add UHS-II module and add a kernel configuration

This patch adds sdhci-uhs2.c as a module for UHS-II support.
This is a skeleton for further development in this patch series.

This kernel configuration, CONFIG_MMC_SDHCI_UHS2, will be used
in the following commits to indicate UHS-II specific code in sdhci
controllers.

Signed-off-by: Ben Chuang <ben.chuang@genesyslogic.com.tw>
Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Signed-off-by: Victor Shih <victor.shih@genesyslogic.com.tw>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Message-ID: <20241018105333.4569-4-victorshihgli@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>