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 <[email protected]>
Signed-off-by: Drew Fustini <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Ulf Hansson <[email protected]>

Author: pdp7