mmc: sdhci-of-dwcmshc: Add support for T-Head TH1520

Add support for the mmc controller in the T-Head TH1520 with the new
compatible "thead,th1520-dwcmshc". Implement custom sdhci_ops for
set_uhs_signaling, reset, voltage_switch, and platform_execute_tuning.

Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Link: https://lore.kernel.org/r/20231109-th1520-mmc-v5-3-018bd039cf17@baylibre.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>