Documentation: synopsys-dw-mshc: remove the unused properties

"support-highspeed" was the obsoleted property.
And "broken-cd" is not synopsys specific property.
It can be referred to mmc.txt binding Documentation.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>