mmc: dt-bindings: add ZTE ZX296718 MMC bindings

Document the device-tree binding of ZTE MMC host on
ZX296718 SoC.

Signed-off-by: Jun Nie <jun.nie@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>