dt-bindings: mmc: synopsys-dw-mshc: document "altr,sysmgr-syscon"

Document the optional "altr,sysmgr-syscon" binding that is used to
access the System Manager register that controls the SDMMC clock
phase.

Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221114230217.202634-1-dinguyen@kernel.org


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>