Documentation: synopsys-dw-mshc: add bindings for idmac and edmac

synopsys-dw-mshc supports three types of transfer mode. We add
bindings and description for how to use them at runtime.

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