dt-bindings: drop msm_sdhci binding

The upstream DT is supported here, so drop the U-Boot specific binding
docs.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>

doc/device-tree-bindings/mmc/msm_sdhci.txt

-Qualcomm Snapdragon SDHCI controller
-
-Required properties:
-- compatible : "qcom,sdhci-msm-v4"
-- reg: Base address and length of registers:
-	- Host controller registers (SDHCI)
-	- SD Core registers
-- clock: interface clock (must accept SD bus clock as a frequency)
-
-Optional properties:
-- index: If there is more than one controller - controller index (required
-	by generic SDHCI code).
-- bus_width: Width of SD/eMMC bus (default 4)
-- clock-frequency: Frequency of SD/eMMC bus (default 400 kHz)
-
-Example:
-
-sdhci@07864000 {
-	compatible = "qcom,sdhci-msm-v4";
-	reg = <0x7864900 0x11c 0x7864000 0x800>;
-	index = <0x1>;
-	bus-width = <0x4>;
-	clock = <&clkc 1>;
-	clock-frequency = <200000000>;
-};