dt-bindings: mmc: sdhci-of-dwcmhsc: Add T-Head TH1520 support

Add compatible value for the T-Head TH1520 dwcmshc controller.

Acked-by: Guo Ren <guoren@kernel.org>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Link: https://lore.kernel.org/r/20231109-th1520-mmc-v5-1-018bd039cf17@baylibre.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>