dt-bindings: mmc: add binding for BST DWCMSHC SDHCI controller

dt-bindings: mmc: add binding for BST DWCMSHC SDHCI controller

Add device tree bindings for the Black Sesame Technologies
DWCMSHC SDHCI controller used in C1200 SoC.

The binding describes a Synopsys DesignWare Cores Mobile Storage
Host Controller with BST-specific extensions including:
- Two register regions (core SDHCI and CRM registers)
- Optional memory-region for bounce buffer support
- Fixed clock input

Signed-off-by: Ge Gordon <gordon.ge@bst.ai>
Signed-off-by: Albert Yang <yangzh0906@thundersoft.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>