dt-bindings: ufs: add common platform bindings

Add bindings for common parts (platform) of Universal Flash Storage
(UFS) Host Controllers in dtschema format.

Include also the bindings directory in the UFS maintainers entry.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220306111125.116455-2-krzysztof.kozlowski@canonical.com