mmc: sdhci-tegra: Add Device Tree probing support

Add hooks to read gpio configuration out of the device tree node.

[grant.likely: Rewrite of original patch from John Bonesio]
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
[swarren: Fixed tegra_sdhci_get_ro() to retrieve pdata correctly]
[swarren: Reworked to avoid #ifdef CONFIG_OF]
[swarren: Reworked binding based on fsl-imx-esdhc.txt]
[swarren: Documented binding]
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>

Documentation/devicetree/bindings/mmc/nvidia-sdhci.txt

@@ -0,0 +1,25 @@
+* NVIDIA Tegra Secure Digital Host Controller
+
+This controller on Tegra family SoCs provides an interface for MMC, SD,
+and SDIO types of memory cards.
+
+Required properties:
+- compatible : Should be "nvidia,<chip>-sdhci"
+- reg : Should contain eSDHC registers location and length
+- interrupts : Should contain eSDHC interrupt
+
+Optional properties:
+- cd-gpios : Specify GPIOs for card detection
+- wp-gpios : Specify GPIOs for write protection
+- power-gpios : Specify GPIOs for power control
+
+Example:
+
+sdhci@c8000200 {
+	compatible = "nvidia,tegra20-sdhci";
+	reg = <0xc8000200 0x200>;
+	interrupts = <47>;
+	cd-gpios = <&gpio 69 0>; /* gpio PI5 */
+	wp-gpios = <&gpio 57 0>; /* gpio PH1 */
+	power-gpios = <&gpio 155 0>; /* gpio PT3 */
+};