Add basic support for the T-Head TH1520 SoC mmc controller with the new
compatible "thead,th1520-dwcmshc".

However, quirks are currently set to disable DMA and use PIO. The proper
settings for DMA support still need to be determined.

Another issue is th1520-specific code for MMC_TIMING_MMC_HS400 in
dwcmshc_set_uhs_signaling() will run on all platforms which is not
correct. One solution could be to add a th1520 flag to dwcmshc_priv but
that is hacky. Another solution could be to set the set_uhs_signaling op
in sdhci_dwcmshc_th1520_ops to a th1520-specific function. However, that
new function would have to duplicate all the code in the current
dwcmshc_set_uhs_signaling().

Signed-off-by: Drew Fustini <dfustini@baylibre.com>